Wsign-compare: Fixed all sign compare warnings in src

This commit addresses all warnings emitted from -Wunused-function
in the C++ src.

Signed-off-by: Noel Eck <noel.eck@intel.com>

src/mic/mic.cxx

@@ -84,7 +84,7 @@ int
 Microphone::findThreshold (thresholdContext* ctx, unsigned int threshold,
                                 uint16_t * buffer, int len) {
     long sum = 0;
-    for (unsigned int i = 0; i < len; i++) {
+    for (int i = 0; i < len; i++) {
         sum += buffer[i];
     }