AP_HAL_Linux: move defines of GPIO LOW/HIGH into cpp file

Insane having these generic defines happening if you include GPIO_BBB.h

libraries/AP_HAL_Linux/GPIO_BBB.cpp

@@ -17,6 +17,9 @@
 #include <sys/mman.h>
 #include <sys/stat.h>
 
+#define LOW             0
+#define HIGH            1
+
 using namespace Linux;
 
 GPIO_BBB::GPIO_BBB()

libraries/AP_HAL_Linux/GPIO_BBB.h

@@ -20,8 +20,6 @@
 #define LED_BLUE        48
 #define LED_SAFETY      61
 #define SAFETY_SWITCH   116
-#define LOW             0
-#define HIGH            1
 
 #if CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_PXF || \
     CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_ERLEBOARD || \