Use size_t for loop counter to avoid sign comparison warning

src/nonsfi/linux/irt_signal_handling.c

Index: src/nonsfi/linux/irt_signal_handling.c
diff --git a/src/nonsfi/linux/irt_signal_handling.c b/src/nonsfi/linux/irt_signal_handling.c
index deda61260ce6260c8630fa0ed858a810dee8f135..8f3fefd6931614edd2bf947b9ec2483a4bc2bda9 100644
--- a/src/nonsfi/linux/irt_signal_handling.c
+++ b/src/nonsfi/linux/irt_signal_handling.c
@@ -477,7 +477,7 @@ int nacl_exception_clear_flag(void) {
    */
   sigset_t *maskptr = (sigset_t *) &mask;
   sigemptyset(maskptr);
-  for (int a = 0; a < NACL_ARRAY_SIZE(kSignals); a++) {
+  for (size_t a = 0; a < NACL_ARRAY_SIZE(kSignals); a++) {
     if (sigaddset(maskptr, kSignals[a]) != 0)
       abort();
   }