x11/wapanel: convert PULSEAUDIO option to subpackage

[jbeich]

Summary

Trivial case: leaf port and doesn't need USE_*.<subpkg>.

Test Plan

$ poudriere testport -j 132i386 x11/wapanel
[...]
===========================================================================
=>> Checking shared library dependencies
 0x00000001 NEEDED               Shared library: [libc++.so.1]
 0x00000001 NEEDED               Shared library: [libc.so.7]
 0x00000001 NEEDED               Shared library: [libcxxrt.so.1]
 0x00000001 NEEDED               Shared library: [libdl.so.1]
 0x00000001 NEEDED               Shared library: [libgcc_s.so.1]
 0x00000001 NEEDED               Shared library: [libgdk-3.so.0]
 0x00000001 NEEDED               Shared library: [libgdk_pixbuf-2.0.so.0]
 0x00000001 NEEDED               Shared library: [libgio-2.0.so.0]
 0x00000001 NEEDED               Shared library: [libglib-2.0.so.0]
 0x00000001 NEEDED               Shared library: [libgobject-2.0.so.0]
 0x00000001 NEEDED               Shared library: [libgtk-3.so.0]
 0x00000001 NEEDED               Shared library: [libgtk-layer-shell.so.0]
 0x00000001 NEEDED               Shared library: [libm.so.5]
 0x00000001 NEEDED               Shared library: [libpango-1.0.so.0]
 0x00000001 NEEDED               Shared library: [libthr.so.3]
 0x00000001 NEEDED               Shared library: [libwayland-client.so.0]

$ poudriere testport -j 132i386 x11/wapanel~pulseaudio
[...]
=>> Checking shared library dependencies
 0x00000001 NEEDED               Shared library: [libc++.so.1]
 0x00000001 NEEDED               Shared library: [libc.so.7]
 0x00000001 NEEDED               Shared library: [libcxxrt.so.1]
 0x00000001 NEEDED               Shared library: [libgcc_s.so.1]
 0x00000001 NEEDED               Shared library: [libgdk-3.so.0]
 0x00000001 NEEDED               Shared library: [libgdk_pixbuf-2.0.so.0]
 0x00000001 NEEDED               Shared library: [libgobject-2.0.so.0]
 0x00000001 NEEDED               Shared library: [libgtk-3.so.0]
 0x00000001 NEEDED               Shared library: [libm.so.5]
 0x00000001 NEEDED               Shared library: [libpulse.so.0]
 0x00000001 NEEDED               Shared library: [libthr.so.3]

[jbeich]

Landed after prefixing @@subpkg@@ with %%OPT%% to avoid breaking OPT=off builds.