gitignore: ignore split DWARF debug info

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

.gitignore

 cscope.out
 *.o
 *.d
+*.dwo
 *.ko
 *.mod.c
 src/tools/wg