Explicit declare gbm library directory

We are checking gbm system library and cflags using pkg, therefore its
directory ought to be checked in the same way.

I spotted this problem in a Ubuntu 12.04 system where my custom pkg directory
is rather clean, without superfluous libraries. Probably no other developer
noticed this bug before because in the working pkg directory was already being
pulled in by libgestures or any other configuration that uses --libs-only-L
in system.gyp.

BUG=none
TEST=embedded=1 ozone_platform_gbm=1, linking content_shell

Review URL: https://codereview.chromium.org/553793002

Cr-Commit-Position: refs/heads/master@{#293903}

build/linux/system.gyp

@@ -413,6 +413,9 @@
             ],
           },
           'link_settings': {
+            'ldflags': [
+              '<!@(<(pkg-config) --libs-only-L --libs-only-other gbm)',
+            ],
             'libraries': [
               '<!@(<(pkg-config) --libs-only-l gbm)',
             ],