sys/include/net/gnrc/gomach.h: fix cpp preprocessorErrorDirective

Preprocesor fails to evaluate the if condicion on L91-92 because
RTT_FREQUENCY is not defined, and therefore a division by 0 occurs.
TO avoid this replicate the RTT_FREQUENCY undefined warning.

sys/include/net/gnrc/gomach/gomach.h

@@ -85,6 +85,9 @@ extern "C" {
 #define GNRC_GOMACH_SUPERFRAME_DURATION_US (300LU * US_PER_MS)
 #endif
 
+#ifndef RTT_FREQUENCY
+#error "RTT_FREQUENCY undefined."
+#else
 #if ((GNRC_GOMACH_SUPERFRAME_DURATION_US < ((1000LU *US_PER_MS) / RTT_FREQUENCY)) || \
  (GNRC_GOMACH_SUPERFRAME_DURATION_US < (10 *GNRC_GOMACH_CP_DURATION_US)))
 #undef GNRC_GOMACH_SUPERFRAME_DURATION_US
@@ -94,6 +97,7 @@ extern "C" {
 #define GNRC_GOMACH_SUPERFRAME_DURATION_US (10 * GNRC_GOMACH_CP_DURATION_US)
 #endif
 #endif
+#endif
 
 /**
  * @brief The maximum duration of the random period at the end of GoMacH's