Fix support for Vala-generated C header files in subdirectories

The support for Vala-generated C header files which was added in commit
60215e7967743ee35bff99f2a8d9f9b3a47c9e11 has two problems:
 • If a project does not use VAPI files, the whole of $(DIST_COMMON)
   gets included in .gitignore, which is completely incorrect.
 • If $(srcdir) is used as a prefix for the VAPI files and C header
   files, extraction of the header file paths from $(DIST_COMMON) fails.

Fix that by not doing a wildcard filter which depends on a potentially
empty string.

Fixes #16.