quilt: Export QUILT_PC variable in ptest Makefile

quilt: Export QUILT_PC variable in ptest Makefile

The quilt ptest uses a custom Makefile to implement the
"make check" rule, but the ptest Makefile does not export
the variable QUILT_PC, which is user-settable and normally
defaults to ".pc". This causes failures e.g. import.test
with "rm -rf patches/ %{QUILT_PC}/", evaluating to
"rm -rf patches/ /" if the variable is not set by the
Makefile.

These ptests broke in:
  73d6034f6b quilt: update to 0.66
when the upstream quilt developers enabled users to specify
their own QUILT_PC string.

Signed-off-by: Trevor Gamblin <trevor.gamblin@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>