graphics/perceptualdiff: fix build without libomp

CMake Error at /usr/local/share/cmake/Modules/FindOpenMP.cmake:261 (try_compile):

Failed to generate test project build system.

Call Stack (most recent call first):

/usr/local/share/cmake/Modules/FindOpenMP.cmake:537 (_OPENMP_GET_FLAGS)
CMakeLists.txt:25 (find_package)

This warning is for project developers. Use -Wno-dev to suppress it.