CMakeLists.txt: Fix CMAKE_MODULE_PATH related issue.

Append to CMAKE_MODULE_PATH instead of overwriting it, in case it is
non-empty initially (which can happen e.g. if -DCMAKE_TOOLCHAIN_FILE is
used and the specified toolchain file sets CMAKE_MODULE_PATH).

Also, append to CMAKE_MODULE_PATH _after_ running "project(pulseview)",
since the latter would apparently reset CMAKE_MODULE_PATH again.

This should fix a MinGW-w64 build issue we're currently seeing:

  CMake Error at CMakeLists.txt:105 (include):
    include could not find load file:

      memaccess