Fix typo in CMakeLists.txt

To compare strings, `STREQUAL` should be used, not `EQUAL`. This
prevented some inaccurate GCC warnings to be considered as non-errors.

CMakeLists.txt

@@ -55,7 +55,7 @@ include_directories("${QEMU_INSTALL_PATH}/include/")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall -Werror -Wno-error=unused-variable")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-error=return-type")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-error=unused-function")
-if(CMAKE_CXX_COMPILER_ID EQUAL "GNU")
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-error=unused-but-set-variable")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-error=maybe-uninitialized")
 endif()