modp_b64: Fix BUILD.gn to be able to build the library with GN

Added missing config and fixed include paths.

BUG=None

Change-Id: I7066a2dfc45b169f1216cdf36addc80897917d15
diff --git a/BUILD.gn b/BUILD.gn
index cd322ac..11c3127 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2,14 +2,17 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+config("modp_b64_config") {
+  include_dirs = [ "." ]
+}
+
 source_set("modp_b64") {
   sources = [
     "modp_b64.cc",
-    "modp_b64.h",
+    "modp_b64/modp_b64.h",
     "modp_b64_data.h",
   ]
 
-  include_dirs = [
-    "./modp_b64",
-  ]
+  public_configs = [ ":modp_b64_config" ]
+  include_dirs = [ "modp_b64" ]
 }