Add forward declarations of Arduino SPI and Wire

A possible solution to #3

Author: oclyke

src/ICM_20948.h

@@ -95,6 +95,11 @@ class ICM_20948 {
 
 
 // I2C
+
+// Forward declarations of TwoWire and Wire for board/variant combinations that don't have a default 'SPI'
+class TwoWire;
+extern TwoWire Wire;
+
 class ICM_20948_I2C : public ICM_20948 {
 private:
 protected:
@@ -124,6 +129,10 @@ class ICM_20948_I2C : public ICM_20948 {
 #define ICM_20948_SPI_DEFAULT_ORDER MSBFIRST
 #define ICM_20948_SPI_DEFAULT_MODE SPI_MODE3
 
+// Forward declarations of SPIClass and SPI for board/variant combinations that don't have a default 'SPI'
+class SPIClass;
+extern SPIClass SPI;
+
 class ICM_20948_SPI : public ICM_20948 {
 private:
 protected: