av1_cx_iface.c: fix signed/unsigned warning

In Visual Studio / cl.exe builds after:
 32c4161373 Use g_bit_depth during input validation

```
av1\av1_cx_iface.c(805,30):
  warning C4018: '>': signed/unsigned mismatch
```

Change-Id: Ic32fdc297162539dc8d5e83fe7d829fb7ecb2e76

av1/av1_cx_iface.c

diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index e33e73d..39d6f15 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -802,7 +802,7 @@
   RANGE_CHECK_HI(extra_cfg, cq_level, 63);
   RANGE_CHECK(cfg, g_bit_depth, AOM_BITS_8, AOM_BITS_12);
   RANGE_CHECK(cfg, g_input_bit_depth, 8, 12);
-  if (cfg->g_input_bit_depth > cfg->g_bit_depth)
+  if (cfg->g_input_bit_depth > (unsigned int)cfg->g_bit_depth)
     ERROR("Input bit-depth must not exceed codec bit-depth");
   RANGE_CHECK(extra_cfg, content, AOM_CONTENT_DEFAULT, AOM_CONTENT_INVALID - 1);