[pkg] don't hardcode full path to sigcheck.exe

the installation path depends on how sigcheck was
installed, in case it is installed from choco its
put in another location

this assumes sigcheck.exe will be available on path

Makefile

@@ -182,7 +182,7 @@ endif
 
 checksign:
 ifeq (${PLATFORM}, windows)
-	@"c:\windows\system32\sigcheck.exe" ${QTBUILD}/release/${TARGET}.exe
+	@sigcheck.exe ${QTBUILD}/release/${TARGET}.exe
 endif
 
 installer: check_qtifw checksign