Fix MinGW build

src/lib/external/kiss_fft/CMakeLists.txt

 # Create a static library from kiss_fft
 set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --std=c99")
+find_package(Qt5Core REQUIRED)
 include_directories(
   ${CMAKE_BINARY_DIR}
 )
@@ -8,3 +9,4 @@ add_library(kiss_fft STATIC
   kiss_fft.c
   tools/kiss_fftr.c
 )
+target_link_libraries(kiss_fft Qt5::Core)