build(msvc): always assume source files are in UTF-8

Fixes https://github.com/ARMmbed/mbedtls/issues/4205

Signed-off-by: Andrea Pappacoda <andrea@pappacoda.it>
(cherry picked from commit 9202909d071e708770fc61437d11e3a9be2b04b9)
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 14ca7b6..9185d4f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -226,8 +226,8 @@
 endif(CMAKE_COMPILER_IS_IAR)
 
 if(CMAKE_COMPILER_IS_MSVC)
-    # Strictest warnings
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /W3")
+    # Strictest warnings, UTF-8 source and execution charset
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /W3 /utf-8")
 endif(CMAKE_COMPILER_IS_MSVC)
 
 if(MBEDTLS_FATAL_WARNINGS)