smart_makepatch: Quote PATCH_WRKSRC when regenerating patches.

Put ${PATCH_WRSKRC} within quotes so that expanding it works properly when it
contains spaces. This is required for `make makepatch' to work with
audio/quimup or any other port that has WRKSRC with spaces. Before the patch:

% make -dl makepatch
cd: too many arguments
cd: too many arguments

and the port would be left with an empty files/ directory.

Reviewed by: marino
Approved by: portmgr (mat)
Differential Revision: https://reviews.freebsd.org/D5011