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>
diff --git a/arch/m68k/lib/string.c b/arch/m68k/lib/string.c
index b9a57ab..4d61fa8 100644
--- a/arch/m68k/lib/string.c
+++ b/arch/m68k/lib/string.c
@@ -17,6 +17,6 @@
 
 char *strcat(char *dest, const char *src)
 {
-	return __kernel_strcpy(dest + __kernel_strlen(dest), src);
+	return __kernel_strcpy(dest + strlen(dest), src);
 }
 EXPORT_SYMBOL(strcat);