Some samd variants are missing SDA and SCL symbols

[bperrybap]

Some of the variants are missing SDA and SCL symbols in the associated variant.h header file.
These variants are missing the symbols:

• mkrfox1200
• mkrgsm1300
• mkrwan1400
• mkrzero

The other variants handle this by creating some constants:

static const uint8_t SDA = PIN_WIRE_SDA;
static const uint8_t SCL = PIN_WIRE_SCL;

Just after the PIN_WIRE_SDA and PIN_WIRE_SDA defines

The lack of these base/required symbols causes some arduino sketch/library s/w to fail to compile.