Fix build warnings

Change-Id: Ic43bcca166a529a6431711b05a7fa21849b6a38b
diff --git a/services/audioflinger/test-resample.cpp b/services/audioflinger/test-resample.cpp
index 3b66530..b082e8c 100644
--- a/services/audioflinger/test-resample.cpp
+++ b/services/audioflinger/test-resample.cpp
@@ -177,7 +177,7 @@
             double t = double(i) / input_freq;
             double y = sin(M_PI * k * t * t);
             int16_t yi = floor(y * 32767.0 + 0.5);
-            for (size_t j=0 ; j<channels ; j++) {
+            for (size_t j=0 ; j<(size_t)channels ; j++) {
                 in[i*channels + j] = yi / (1+j);
             }
         }