cmake: Fix cross-compilation to windows.

The change committed in 6012678 causes a fatal error if the ffmpeg
applications can't be found and used to determine its feature set.
This is fine for local compiles but cross compiled executable can't be
executed, so there's no point in requiring their existence (at this
point in the compilation).  Exempt cross compilation from this new
check.

mythtv/cmake/MythFindPackages.cmake

@@ -24,8 +24,9 @@ find_package(PkgConfig REQUIRED)
 # Packages provided by MythTV.  These have no dependencies on MythTV and must
 # have already been compiled and installed by the super-package.
 #
-if(CMAKE_SYSTEM_NAME MATCHES "Linux|Windows" AND CMAKE_SYSTEM_PROCESSOR MATCHES
-                                                 "x86|aarch64")
+if(NOT CMAKE_CROSSCOMPILING
+   AND CMAKE_SYSTEM_NAME MATCHES "Linux|Windows"
+   AND CMAKE_SYSTEM_PROCESSOR MATCHES "x86|aarch64")
   pkg_check_modules(FFNVCODEC ffnvcodec REQUIRED IMPORTED_TARGET)
   add_build_config(PkgConfig::FFNVCODEC "nvdec")
   if(TARGET PkgConfig::FFNVCODEC)