keep util/build_version.cc when make clean

Summary:
https://github.com/facebook/rocksdb/pull/2264
adding build_version.cc into clean list which breaks fbcode release.
we need to keep it when `make clean`
Closes https://github.com/facebook/rocksdb/pull/2322

Differential Revision: D5088932

Pulled By: lightmark

fbshipit-source-id: ab001424af596e94a6bc1d4186c39edf6ace484f

Makefile

@@ -304,7 +304,6 @@ util/build_version.cc: FORCE
 	  cmp -s $@-t $@ && rm -f $@-t || mv -f $@-t $@;		\
 	else mv -f $@-t $@; fi
 endif
-CLEAN_FILES += util/build_version.cc
 
 LIBOBJECTS = $(LIB_SOURCES:.cc=.o)
 LIBOBJECTS += $(TOOL_LIB_SOURCES:.cc=.o)