m68k: Remove inline strlen() implementation

GCC can replace a strncat() call with constant second argument into a
strlen + store, which results in a link error:

ERROR: "strlen" [net/ipv4/ip_tunnel.ko] undefined!

The inline function is a simple for loop in C. Other architectures
either use an asm optimized variant, or use the generic function from
lib/string.c.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>