lib: mbedtls: CMakeLists.txt: fix a typo for CFLAGS (#2662)

CMAKE_C_CFLAGS -> CMAKE_C_FLAGS, this avoids C_FLAGS from environment
being overridden.

Signed-off-by: Ming Liu <liu.ming50@gmail.com>

Author: liuming50

lib/mbedtls-2.24.0/CMakeLists.txt

@@ -154,7 +154,7 @@ string(REGEX MATCH "Clang" CMAKE_COMPILER_IS_CLANG "${CMAKE_C_COMPILER_ID}")
 
 include(CheckCCompilerFlag)
 
-set(CMAKE_C_FLAGS "${CMAKE_C_CFLAGS} -std=c99")
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99")
 
 if(CMAKE_COMPILER_IS_GNU)
     # some warnings we want are not available with old GCC versions