Fix building usr.bin/tar/tests with PIE symbol building enabled by
removing CFLAGS+= -static

CFLAGS+= -static was a carryover from pre-r289195 with
usr.bin/tar/test/Makefile that should have been specified in LDFLAGS
There doesn't seem to be an apparent need for static compilation
of the test binaries.

Differential Revision: https://reviews.freebsd.org/D7430
MFC after: 1 week
Obtained-from: opBSD (418a491eed20d2603ddd1f1bd92c2c0d95094002)
Submitted by: op
Sponsored by: EMC / Isilon Storage Division