Find and link Qt5Widgets for PYBINDINGS.

At least one PYBINDINGS module requires linking with Qt5Widgets:
pytraffic. If we don't find this library, it won't be linked, and thus
pytraffic won't work at all.

CMakeLists.txt

@@ -182,7 +182,7 @@ if (NOT PLATFORM_IPHONE AND NOT PLATFORM_ANDROID)
     message(FATAL_ERROR "Minimum supported Qt5 version is 5.5")
   endif()
   find_package(Qt5Network REQUIRED)
-  if (NOT SKIP_DESKTOP OR NOT SKIP_TESTS)
+  if (NOT SKIP_DESKTOP OR NOT SKIP_TESTS OR PYBINDINGS)
     find_package(Qt5Widgets)
   endif()
   if (NOT SKIP_DESKTOP)