Fix typedef of sig_t on AIX (#1030)

Author: miladfarca

Diff for: absl/time/clock_test.cc

@@ -18,8 +18,10 @@
 #if defined(ABSL_HAVE_ALARM)
 #include <signal.h>
 #include <unistd.h>
-#elif defined(_AIX)
+#ifdef _AIX
+// sig_t is not defined in AIX.
 typedef void (*sig_t)(int);
+#endif
 #elif defined(__linux__) || defined(__APPLE__)
 #error all known Linux and Apple targets have alarm
 #endif