[DOC] Exclude require in RubyGems not to override the original

Fix ruby/rdoc#1592.

Author: nobu

common.mk

@@ -75,6 +75,7 @@ HTMLOUT       = $(EXTOUT)/html
 CAPIOUT       = doc/capi
 INSTALL_DOC_OPTS = --rdoc-output="$(RDOCOUT)" --html-output="$(HTMLOUT)"
 RDOC_GEN_OPTS = --no-force-update \
+	--exclude '^lib/rubygems/core_ext/kernel_require\.rb$$' \
 	$(empty)
 
 INITOBJS      = dmyext.$(OBJEXT) dmyenc.$(OBJEXT)