[farmhash] Set include_dirs in config

... so dependents can import farmhash.h directly. This makes it possible
to move farmhash without breaking dependents.

Change-Id: Icf7b35bae764b0221ea9f8e0b407c2e7e0857c6c
diff --git a/BUILD.gn b/BUILD.gn
index 75cb50a..70bac12 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -5,6 +5,7 @@
 config("farmhash_config") {
   defines = [ "NAMESPACE_FOR_HASH_FUNCTIONS=farmhash" ]
   cflags = [ "-Wno-conversion" ]
+  include_dirs = [ "src" ]
 }
 
 source_set("farmhash") {