add static initialization for pthread mutexes

The documentation says:
"In cases where default mutex attributes are appropriate, the macro
PTHREAD_MUTEX_INITIALIZER can be used to initialize mutexes that are
statically allocated. The effect shall be equivalent to dynamic
initialization by a call to pthread_mutex_init() with parameter attr
specified as NULL, except that no error checks are performed."

utility/RPi/interrupt.c

@@ -23,7 +23,7 @@ see <http://www.gnu.org/licenses/>
 
 #define delay(x) bcm2835_delay(x)
 
-static pthread_mutex_t pinMutex ;
+static pthread_mutex_t pinMutex = PTHREAD_MUTEX_INITIALIZER;
 static volatile int    pinPass = -1 ;
 
 pthread_t threadId [64];

utility/RPi/spi.cpp

 #include "spi.h"
 #include <pthread.h>
 
-static pthread_mutex_t spiMutex;
+static pthread_mutex_t spiMutex = PTHREAD_MUTEX_INITIALIZER;
 
 SPI::SPI() {