Add fmt prefix to includes (#332)

Technically it is not necessary, but prevents errors when people add the
fmt directory itself rather than its parent to includes.

test/CMakeLists.txt

@@ -111,6 +111,7 @@ endif ()
 check_cxx_compiler_flag(-fno-exceptions HAVE_FNO_EXCEPTIONS_FLAG)
 if (HAVE_FNO_EXCEPTIONS_FLAG)
   add_library(noexception-test ../fmt/format.cc)
+  target_include_directories(noexception-test PRIVATE ${PROJECT_SOURCE_DIR})
   target_compile_options(noexception-test PRIVATE -fno-exceptions)
 endif ()
 
@@ -118,6 +119,7 @@ if (FMT_PEDANTIC)
   # Test that the library compiles without windows.h.
   if (CMAKE_SYSTEM_NAME STREQUAL "Windows")
     add_library(no-windows-h-test ../fmt/format.cc)
+    target_include_directories(no-windows-h-test PRIVATE ${PROJECT_SOURCE_DIR})
     target_compile_definitions(no-windows-h-test PRIVATE FMT_USE_WINDOWS_H=0)
   endif ()