Makefile: Add pkcs7 library for building the Win32 viewer.
Makefile: Add pkcs7 library for building the Win32 viewer.

Commit 0b04079f5057ba6c7726bd14b61abed9fde1957a upstream abstracts the
pkcs7 code into a helper library, and adds that library to the mutool
and mupdf-x11 builds. This commit adds the library to the Win32 viewer
build, which relies on the same functionality as mupdf-x11 (through
pdfapp.c).

TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf