function declarations shouldn't be marked inline

GCC 6.3 in Debian Strech uses C99 inline semantics, so it complains:

    In file included from eeprom_local.c:31:0:
    eeprom_local.c: At top level:
    epp_boards.h:37:13: warning: inline function ‘epp_write8’ declared but never defined
     inline void epp_write8(board_t *board, u8 data);
		 ^~~~~~~~~~

The function declaration needs to be non-inline, and the function
definition can be inline.