gdb/Makefile.in: fix 'make tags' build target

gdb/Makefile.in: fix 'make tags' build target

The gdb_select.h file was moved to the gdbsupport directory long ago,
but a reference was accident left in gdb/Makefile.in (in the
HFILES_NO_SRCDIR variable), this commit removes that reference.

Before this commit, if I use 'make tags' here's what I see:

  $ make tags
  make: *** No rule to make target 'gdb_select.h', needed by 'TAGS'.  Stop.

After this commit 'make tags' completes, but I still see these
warnings:

  ../../src/gdb/arm.c: No such file or directory
  ../../src/gdb/arm-get-next-pcs.c: No such file or directory
  ../../src/gdb/arm-linux.c: No such file or directory

These are caused by a separate issue, and will be addressed in the
next commit.