VP8_COMP: make frames_since_golden signed

This value is signed in vp9/10

Cleans warning in Android build:
comparison of integers of different signs: 'unsigned int' and 'int'
if (cpi->frames_since_golden == (cpi->current_gf_interval >> 1))
    ~~~~~~~~~~~~~~~~~~~~~~~~ ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Change-Id: Ie137724982f3a46c8c1820548c1960d62a4e96f2

vp8/encoder/onyx_int.h

@@ -371,7 +371,7 @@ typedef struct VP8_COMP
     double key_frame_rate_correction_factor;
     double gf_rate_correction_factor;
 
-    unsigned int frames_since_golden;
+    int frames_since_golden;
     /* Count down till next GF */
     int frames_till_gf_update_due;