build,cpp: Fix include paths in forwarding headers in gpgme++ folder

* lang/cpp/src/Makefile.am (build rule for copied headers): Write
abs_srcdir instead of srcdir in the generated headers.

The generated include paths lacked a "../" if srcdir was relative. This
lead to self-including includes for in-source builds. Including the real
headers with their absolute paths fixes this.

• GnuPG-bug-id: T7110