Support llvmc plugins in out-of-tree projects.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61990 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/Makefile.rules b/Makefile.rules
index ca7b802..33dcbda 100644
--- a/Makefile.rules
+++ b/Makefile.rules
@@ -517,6 +517,7 @@
 # paths. In this case, the SYSPATH function (defined in
 # Makefile.config) transforms Unix paths into Windows paths.
 TableGen      = $(TBLGEN) -I $(call SYSPATH, $(PROJ_SRC_DIR)) \
+                -I $(call SYSPATH, $(LLVM_SRC_ROOT)/include) \
                 -I $(call SYSPATH, $(PROJ_SRC_ROOT)/include) \
                 -I $(call SYSPATH, $(PROJ_SRC_ROOT)/lib/Target)