Don't fail if llvm.mk is not available.

This enables doing dalvik minimal host build without external/llvm.

Change-Id: I1f44f4d29b941086a79758c816674c9f9966eb31
diff --git a/build/Android.common.mk b/build/Android.common.mk
index ac1be1e..dd0ba4d 100644
--- a/build/Android.common.mk
+++ b/build/Android.common.mk
@@ -82,7 +82,8 @@
 endif
 
 LLVM_ROOT_PATH := external/llvm
-include $(LLVM_ROOT_PATH)/llvm.mk
+# Don't fail a dalvik minimal host build.
+-include $(LLVM_ROOT_PATH)/llvm.mk
 
 # Clang build.
 # ART_TARGET_CLANG := true