zephyr: Fix mp_hal_set_interrupt_char() declaration to be compatible.

With other ports. Other ports declare it in mphalport.h, it can be
inline or macro.

lib/utils/pyexec.c

@@ -53,8 +53,6 @@ STATIC bool repl_display_debugging_info = 0;
 #define EXEC_FLAG_IS_REPL (4)
 #define EXEC_FLAG_SOURCE_IS_RAW_CODE (8)
 
-extern void mp_hal_set_interrupt_char (int c);
-
 // parses, compiles and executes the code in the lexer
 // frees the lexer before returning
 // EXEC_FLAG_PRINT_EOF prints 2 EOF chars: 1 after normal output, 1 after exception output