Fix a linkage problem with the previous patch for "thr_client_alarm".

mysys/my_pthread.c:
  Linkage problem with previous patch: "thr_client_alarm" must be declared in here.
mysys/thr_alarm.c:
  Linkage problem: Declare "thr_client_alarm" over in "mysys/my_pthread.c".

mysys/my_pthread.c

@@ -32,6 +32,7 @@
 #endif
 
 uint thd_lib_detected;
+uint thr_client_alarm;
 
 #ifndef my_pthread_setprio
 void my_pthread_setprio(pthread_t thread_id,int prior)

mysys/thr_alarm.c

@@ -37,7 +37,6 @@
 #define ETIME ETIMEDOUT
 #endif
 
-uint thr_client_alarm;
 static int alarm_aborted=1;			/* No alarm thread */
 my_bool thr_alarm_inited= 0;
 volatile my_bool alarm_thread_running= 0;