Do not regenerate revision.h

Ruby's makefile compares the shipped version with the git revision to regenerate
revision.h [1], but since we don't include git in buildInputs, this comparison
fails and it can't find $(REVISION_H).

[1] https://github.com/ruby/ruby/commit/97a5af62a318fcd93a4e5e4428d576c0280ddbae

diff -Naur ruby.old/defs/gmake.mk ruby.new/defs/gmake.mk
--- ruby.old/defs/gmake.mk
+++ ruby.new/defs/gmake.mk
@@ -325,11 +325,9 @@
 
 REVISION_IN_HEADER := $(shell sed -n 's/^\#define RUBY_FULL_REVISION "\(.*\)"/\1/p' $(srcdir)/revision.h 2>/dev/null)
 REVISION_LATEST := $(shell $(CHDIR) $(srcdir) && git log -1 --format=%H 2>/dev/null)
-ifneq ($(REVISION_IN_HEADER),$(REVISION_LATEST))
 # GNU make treat the target as unmodified when its dependents get
 # updated but it is not updated, while others may not.
 $(srcdir)/revision.h: $(REVISION_H)
-endif
 
 # Query on the generated rdoc
 #