Fix failure of make distclean when linuxkm is enabled

$ make distclean
Making distclean in linuxkm
make[1]: Entering directory '/home/honma/git/wolfssl/linuxkm'
make[1]: *** No rule to make target 'distclean'.  Stop.
make[1]: Leaving directory '/home/honma/git/wolfssl/linuxkm'
make: *** [Makefile:6431: distclean-recursive] Error 1

Signed-off-by: Masashi Honma <masashi.honma@gmail.com>