aomenc.c: add missing cast

Fixes a `-Wsign-compare` warning seen with clang 18.1.8 shipped with
Microsoft Visual Studio 2022 17.12.4.

aomenc.c:2321:76: warning: comparison of integers of different signs:
  'unsigned int' and 'aom_bit_depth_t' (aka 'enum aom_bit_depth')
  [-Wsign-compare]
| if ((global.show_psnr == 2) && (stream->config.cfg.g_input_bit_depth ==
|                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
|                                 stream->config.cfg.g_bit_depth)) {
|                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Change-Id: Ifb30405efe6b7e4c9cba72922ef79b2e635c4084

apps/aomenc.c

diff --git a/apps/aomenc.c b/apps/aomenc.c
index ff60ef0..3d58a89 100644
--- a/apps/aomenc.c
+++ b/apps/aomenc.c
@@ -2318,8 +2318,9 @@
                 "match input format.\n",
                 stream->config.cfg.g_profile);
       }
-      if ((global.show_psnr == 2) && (stream->config.cfg.g_input_bit_depth ==
-                                      stream->config.cfg.g_bit_depth)) {
+      if (global.show_psnr == 2 &&
+          stream->config.cfg.g_input_bit_depth ==
+              (unsigned int)stream->config.cfg.g_bit_depth) {
         fprintf(stderr,
                 "Warning: --psnr==2 and --psnr==1 will provide same "
                 "results when input bit-depth == stream bit-depth, "