kbuild: fix cc-version

cc-version needs to use $(shell to get the gcc version.
Before if gave the following error when building the kernel:

/bin/sh: line 1: [: too many arguments

And all checks for gcc version were broken.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -279,8 +279,8 @@ cc-option-yn = $(shell if $(CC) $(CFLAGS) $(1) -S -o /dev/null -xc /dev/null \
 
 # cc-version
 # Usage gcc-ver := $(call cc-version $(CC))
-cc-version = $(CONFIG_SHELL) $(srctree)/scripts/gcc-version.sh \
-              $(if $(1), $(1), $(CC))
+cc-version = $(shell $(CONFIG_SHELL) $(srctree)/scripts/gcc-version.sh \
+              $(if $(1), $(1), $(CC)))
 
 
 # Look for make include files relative to root of kernel src