Mk/Uses/ssl.mk: Announce OpenSSL in base to LOCALBASE using PKGCONFIG_BASE

[diizzy]

Summary

Now that we have pkgconfig files for OpenSSL in base utilize PKGCONFIG_BASE to annouce these in LOCALBASE

OSVERSION reflects on which support for pkgconfig were committed for each branch so next version bump contains these files for sure.

Reference:
13-STABLE
https://cgit.freebsd.org/src/commit/secure/lib/libssl/openssl.pc.in?h=stable/13&id=41067961b6f3a0b5667e40e768b7b46e4092348e
https://cgit.freebsd.org/src/tree/sys/sys/param.h?h=stable/13&id=41067961b6f3a0b5667e40e768b7b46e4092348e#n63
14-CURRENT
https://cgit.freebsd.org/src/commit/secure/lib/libssl/openssl.pc.in?id=b323fa85f15268ac44b8ff90faf90bce5d87b608
https://cgit.freebsd.org/src/tree/sys/sys/param.h?id=b323fa85f15268ac44b8ff90faf90bce5d87b608#n79

[tcberner]

Mk/Uses/ssl.mk

86

this should possibly be += as a port using ssl.mk might already have its own PKGCONFIG_BASE values set.

[diizzy]

Mk/Uses/ssl.mk

86

True, anything else we should consider?