[posix] fix the compile errors of src/posix/platform/alarm.cpp (#11658)

The definition of sMicroTimer and sRealTimeSignal are wrapped by the
__linux__, OPENTHREAD_CONFIG_PLATFORM_USEC_TIMER_ENABLE and
!OPENTHREAD_POSIX_VIRTUAL_TIME.  But the code using these two
variables is not completely wrapped by these three macros. It causes
the compile errors in some conditions.

This commit wraps all the code that uses these two variables with
these three macros.

src/posix/platform/alarm.cpp

@@ -173,7 +173,7 @@ void otPlatAlarmMicroStartAt(otInstance *aInstance, uint32_t aT0, uint32_t aDt)
     sUsAlarm     = aT0 + aDt;
     sIsUsRunning = true;
 
-#ifdef __linux__
+#if defined(__linux__) && !OPENTHREAD_POSIX_VIRTUAL_TIME
     if (sRealTimeSignal != 0)
     {
         struct itimerspec its;
@@ -190,7 +190,7 @@ void otPlatAlarmMicroStartAt(otInstance *aInstance, uint32_t aT0, uint32_t aDt)
             otLogWarnPlat("Failed to update microsecond timer: %s", strerror(errno));
         }
     }
-#endif // __linux__
+#endif // defined( __linux__) && !OPENTHREAD_POSIX_VIRTUAL_TIME
 }
 
 void otPlatAlarmMicroStop(otInstance *aInstance)
@@ -199,7 +199,7 @@ void otPlatAlarmMicroStop(otInstance *aInstance)
 
     sIsUsRunning = false;
 
-#ifdef __linux__
+#if defined(__linux__) && !OPENTHREAD_POSIX_VIRTUAL_TIME
     if (sRealTimeSignal != 0)
     {
         struct itimerspec its = {{0, 0}, {0, 0}};
@@ -209,7 +209,7 @@ void otPlatAlarmMicroStop(otInstance *aInstance)
             otLogWarnPlat("Failed to stop microsecond timer: %s", strerror(errno));
         }
     }
-#endif // __linux__
+#endif // defined( __linux__) && !OPENTHREAD_POSIX_VIRTUAL_TIME
 }
 #endif // OPENTHREAD_CONFIG_PLATFORM_USEC_TIMER_ENABLE