graphics/libpano13: fix linking

This fixes the build errors:
[...]
ld: error: /wrkdirs/usr/ports/graphics/libpano13/work/libpano13-2.9.19/.libs/libpano13.so: undefined reference to ceil
ld: error: /wrkdirs/usr/ports/graphics/libpano13/work/libpano13-2.9.19/.libs/libpano13.so: undefined reference to atan
ld: error: /wrkdirs/usr/ports/graphics/libpano13/work/libpano13-2.9.19/.libs/libpano13.so: undefined reference to tan
[...]
by passing -lm to the linker flags for FreeBSD builds.