Fix cmake warning with pthread

[dalg24]

-- Architectures:
CMake Warning (dev) at /Applications/CMake.app/Contents/share/cmake-3.19/Modules/FindPackageHandleStandardArgs.cmake:426 (message):
  The package name passed to `find_package_handle_standard_args` (PTHREAD)
  does not match the name of the calling package (TPLPTHREAD).  This can lead
  to problems in calling code that expects `find_package` result variables
  (e.g., `_FOUND`) to follow a certain pattern.
Call Stack (most recent call first):
  cmake/Modules/FindTPLPTHREAD.cmake:13 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  cmake/kokkos_functions.cmake:235 (FIND_PACKAGE)
  cmake/kokkos_tpls.cmake:72 (KOKKOS_IMPORT_TPL)
  cmake/kokkos_tribits.cmake:236 (INCLUDE)
  CMakeLists.txt:170 (KOKKOS_SETUP_BUILD_ENVIRONMENT)
This warning is for project developers.  Use -Wno-dev to suppress it.

[masterleinad]

Looks OK. We never use PTHREAD_FOUND.