gsm/Makefile.am: use proper top_builddir/top_srcdir paths

This fixes the conv*gen.c targets when building in a different directory
than the source tree.

Notably, building in a different dir worked when the generated sources were
already present from a previous build inside the source directory.

Change-Id: I5a9b780ad4ba607ea39854dcf7207ed05f5447bc
1 file changed