qb: Include /usr/local/include for bsd based systems.

This resolves an issue where the default freebsd compiler does not
include /usr/local/include which contains important headers such as
GL/gl.h and results in the check_header function failing to find them.

Unfortunately pkg-config/pkgconf will not work here for two reasons.
1. It does not seem able to actually check for gl.h which RetroArch
explicitly needs.

2. Not all systems have a pkg-config implementation so we will still
have to fall back to checking for gl.h...

Fixes https://github.com/libretro/RetroArch/issues/5958

qb/config.libs.sh

@@ -23,6 +23,7 @@ SOCKETLIB=-lc
 SOCKETHEADER=
 
 if [ "$OS" = 'BSD' ]; then
+   [ -d /usr/local/include ] && add_dirs INCLUDE /usr/local/include
    DYLIB=-lc;
 elif [ "$OS" = 'Haiku' ]; then
    DYLIB=""