libsoc: fix compilation error caused by multiple goals on make

libsoc: fix compilation error caused by multiple goals on make

Error of the following kind can happen when parallel build with goals "all" and
"install" both specified on the command line.  Seems like make can parallelly
build for both targets causing race conditions

	make[3]: Entering directory `/tmp/tmp.HIR3tEIqBb/build_dir/target-mips_24kc_musl/libsoc-2016-12-22-5b788d4d'
	Making all in lib
	Making install in lib
	make[4]: Entering directory `/tmp/tmp.HIR3tEIqBb/build_dir/target-mips_24kc_musl/libsoc-2016-12-22-5b788d4d/lib'
	  CC       libsoc_la-gpio.lo
	make[4]: Entering directory `/tmp/tmp.HIR3tEIqBb/build_dir/target-mips_24kc_musl/libsoc-2016-12-22-5b788d4d/lib'
	  CC       libsoc_la-spi.lo
	  CC       libsoc_la-gpio.lo
	  CC       libsoc_la-file.lo
	  CC       libsoc_la-i2c.lo
	  CC       libsoc_la-spi.lo
	  CC       libsoc_la-pwm.lo
	  CC       libsoc_la-board.lo
	  CC       libsoc_la-file.lo
	  CC       libsoc_la-conffile.lo
	  CC       libsoc_la-debug.lo
	  CC       libsoc_la-pwm.lo
	  CC       libsoc_la-conffile.lo
	  CCLD     libsoc.la
	OpenWrt-libtool: link: `libsoc_la-pwm.lo' is not a valid libtool object
	make[4]: *** [libsoc.la] Error 1

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>