kbuild: Move the definition of MODVERDIR

MODVERDIR was defined in the build-only section, but it's needed for
"make mrproper" as well.

Makefile

@@ -183,6 +183,8 @@ export CPPFLAGS NOSTDINC_FLAGS OBJCOPYFLAGS LDFLAGS
 export CFLAGS CFLAGS_KERNEL CFLAGS_MODULE 
 export AFLAGS AFLAGS_KERNEL AFLAGS_MODULE
 
+export MODVERDIR := .tmp_versions
+
 # The temporary file to save gcc -MD generated dependencies must not
 # contain a comma
 depfile = $(subst $(comma),_,$(@D)/.$(@F).d)
@@ -272,8 +274,6 @@ endif
 endif
 endif
 
-export MODVERDIR := .tmp_versions
-
 #
 # INSTALL_PATH specifies where to place the updated kernel and system map
 # images.  Uncomment if you want to place them anywhere other than root.