Resolve adafruit#4 by making gpio mem global explicitly extern in header.

Author: tdicola

source/Raspberry_Pi/pi_mmio.h

@@ -30,7 +30,7 @@
 #define MMIO_ERROR_DEVMEM -1
 #define MMIO_ERROR_MMAP -2
 
-volatile uint32_t* pi_mmio_gpio;
+extern volatile uint32_t* pi_mmio_gpio;
 
 int pi_mmio_init(void);
 

source/Raspberry_Pi_2/pi_2_mmio.h

@@ -31,7 +31,7 @@
 #define MMIO_ERROR_MMAP -2
 #define MMIO_ERROR_OFFSET -3
 
-volatile uint32_t* pi_2_mmio_gpio;
+extern volatile uint32_t* pi_2_mmio_gpio;
 
 int pi_2_mmio_init(void);