Typo may affect CMake build system tests

[zao]

In

hpx/tests/unit/build/CMakeLists.txt

Line 116 in 9303e9a

set(build_systems ${build_sytems} pkgconfig)

set(build_systems cmake)
if(NOT CMAKE_TOOLCHAIN_FILE)
  set(build_systems ${build_sytems} pkgconfig)
endif()

The typo is build_sytems, causing the cmake one to be forgotten for add_hpx_pseudo_target and add_hpx_pseudo_dependencies.