kbuild: move scripts/basic/docproc.c to scripts/docproc.c

Move docproc from scripts/basic to scripts so it is only built for *doc
targets instead of every time the kernel is built.
diff --git a/Makefile b/Makefile
index d342502..bc9eae4 100644
--- a/Makefile
+++ b/Makefile
@@ -1313,6 +1313,7 @@
 # Documentation targets
 # ---------------------------------------------------------------------------
 %docs: scripts_basic FORCE
+	$(Q)$(MAKE) $(build)=scripts build_docproc
 	$(Q)$(MAKE) $(build)=Documentation/DocBook $@
 
 else # KBUILD_EXTMOD