common.mk: remove extra slash

* common.mk (incs): remove extra slash before
  vm_call_iseq_optimized.inc.  as nmake vpath syntax includes a
  separator, don't append it.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52262 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

common.mk

@@ -786,7 +786,7 @@ srcs-enc: $(ENC_MK)
 
 all-incs: incs {$(VPATH)}encdb.h {$(VPATH)}transdb.h
 incs: $(INSNS) {$(VPATH)}node_name.inc {$(VPATH)}known_errors.inc \
-      {$(VPATH)}/vm_call_iseq_optimized.inc $(srcdir)/revision.h \
+      {$(VPATH)}vm_call_iseq_optimized.inc $(srcdir)/revision.h \
       $(REVISION_H) enc/unicode/name2ctype.h enc/jis/props.h \
       {$(VPATH)}id.h {$(VPATH)}probes.dmyh