config/debuginfod: do not include pkg.m4 directly

Any code using AC_DEBUGINFOD from this dir is using -I../config when
running aclocal, so an explicit include on pkg.m4 is unnecessary:
aclocal will find the pkg.m4 in this dir just as easily.  This is
seen in the only two dirs that use AC_DEBUGINFOD (binutils & gdb)
as their aclocal.m4 already has m4_include on config m4 files.

The include as written only works if aclocal is run on a dir that is
at the same level of config/.  Any other depth will fail.
./
|-- config/
|-- binutils/    # works
|-- gdb/         # works
`-- sim/         # works
    `-- <port>/  # fails

It fails even if AC_DEBUGINFOD itself isn't used:
sim/bfin/ $ aclocal -I../../config
aclocal-1.15: error: ../../config/debuginfod.m4:8: file '../config/pkg.m4' does not exist