Fix BUILD.gn file

After headers were moved to modp_b64 folder BUILD.gn file was
not updated to reflect the change.

Change-Id: I619d450193af3bf7256b10f6459ef5825ce12451

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 2ec992b..cd322ac 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -8,4 +8,8 @@
     "modp_b64.h",
     "modp_b64_data.h",
   ]
+
+  include_dirs = [
+    "./modp_b64",
+  ]
 }