m68k: lib: Include <linux/libgcc.h> for __muldi3()

When building with W=1:

    arch/m68k/lib/muldi3.c:82:1: warning: no previous prototype for ‘__muldi3’ [-Wmissing-prototypes]
       82 | __muldi3 (DItype u, DItype v)
	  | ^~~~~~~~

Fix this by including <linux/libgcc.h>.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/160c1fd14b4798f576d9649334b1d2c77db5cb07.1697008341.git.geert@linux-m68k.org