Fix coverage build.

NOTICE: The coverage build is not officially supported and may break
at any point.
Patch receieved from johan.ahlers@gmail.com.

BUG=webrtc:5754
NOTRY=True
TBR=henrika@webrtc.org

Review URL: https://codereview.webrtc.org/1875193002

Cr-Commit-Position: refs/heads/master@{#12420}

webrtc/build/common.gypi

@@ -372,6 +372,7 @@
       ['coverage==1 and OS=="linux"', {
         'cflags': [ '-ftest-coverage',
                     '-fprofile-arcs' ],
+        'ldflags': [ '--coverage' ],
         'link_settings': { 'libraries': [ '-lgcov' ] },
       }],
       ['os_posix==1', {