Correct the path of a header file in source list.

The header file path should be relative to the location of BUILD.gn.
This error is detected by gn gen --check, but it hasn't because the
check option has not been applied by common-mk/platform2.py.

BUG=chromium:1018878
TEST=run "gn gen --check" like done by crrev.com/c/2215951

Change-Id: I3057e18739d82e2212927d6446a359f7665aac89
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/external/modp_b64/+/2215863
Tested-by: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 9107609..aceab6e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -24,7 +24,7 @@
   ]
   sources = [
     "modp_b64.cc",
-    "modp_b64.h",
+    "modp_b64/modp_b64.h",
     "modp_b64_data.h",
   ]
   include_dirs = [