FYI: maint.mk: update regex to reflect 2013 addition of "assume" to verify.h

[Jim Meyering]

Updating grep to use latest gnulib exposed this:
"make syntax-check" would fail with this:

  src/kwset.c
  maint.mk: the above files include verify.h but don't use it
  maint.mk:698: recipe for target 'sc_prohibit_verify_without_use' failed
  make: *** [sc_prohibit_verify_without_use] Error 1

I've pushed the attached:

gnulib-maint.mk-verify-vs-assume.diff
Description: Text document