Log Message:
libgit2: include pcre in buildlink3 file

The pkg-config file for libgit2 specifies pcre as a hard dependency, which
means that "pkg-config --libs libgit2" will exit with an error. This means
that every dependent package also depends on pcre, so add it to the list
of dependencies exposed by buildlink3.

Noticed in the pending wip/cargo-edit package, which swallows the error
message, leaving me to debug in the bowels of the Cargo build system.