including mpsse.h in projects can cause "duplicate symbol" linker errors

[ep00ch]

including mpsse.h in multiple source files can cause "duplicate symbol" linker errors due to fast_rw_buf being defined in the header file.

I removed
unsigned char fast_rw_buf[SPI_RW_SIZE + CMD_SIZE];
from mpsse.h
and added
static unsigned char fast_rw_buf[SPI_RW_SIZE + CMD_SIZE];
to fast.c

Maybe Im not sure how this is supposed to be used, but it only seems useful in fast.c's functions which are "For internal use only"

[BhargavaRamM]

Possible duplicate of #11. You can check out PR #12 which is not yet merged.