kbuild: fix stage 2 of module build

When building allmodconfig the following error were seen:
scripts/Makefile.modpost:86: target `fs/nls/nls_koi8-ru.ko' given more than once in the same rule.
This happened for all modules with 'ko' inside their name.
This bug were introduced when head ... were replaced by grep.

Signed off by: Sam Ravnborg <sam@ravnborg.org>

scripts/Makefile.modpost

@@ -41,7 +41,7 @@ include scripts/Makefile.lib
 symverfile := $(objtree)/Module.symvers
 
 # Step 1), find all modules listed in $(MODVERDIR)/
-__modules := $(sort $(shell grep -h .ko /dev/null $(wildcard $(MODVERDIR)/*.mod)))
+__modules := $(sort $(shell grep -h '\.ko' /dev/null $(wildcard $(MODVERDIR)/*.mod)))
 modules   := $(patsubst %.o,%.ko, $(wildcard $(__modules:.ko=.o)))
 
 _modpost: $(modules)