gdb/Makefile.in: fix 'make tags' failure

gdb/Makefile.in: fix 'make tags' failure

'make tags' fails with the following error:

  make[2]: Entering directory '/local-ssd/mgulick/gdb/git/binutils-gdb/gdb'
  make[2]: *** No rule to make target 'gdb.h', needed by 'TAGS'.  Stop.
  make[2]: Leaving directory '/local-ssd/mgulick/gdb/git/binutils-gdb/gdb'

The file gdb/gdb.h was removed in commit
65630365f7d073430e62b4fe65f34dcdc0a4b05e.

gdb/ChangeLog:

2017-10-30  Mike Gulick  <mgulick@mathworks.com>

	* Makefile.in (HFILES_NO_SRCDIR): Remove reference to gdb.h.