Fix macroization of inline in C++

When compiling as C++, MSVC complains about our macroization of a keyword.
Stop doing that as we know inline is always available in C++
diff --git a/include/polarssl/md.h b/include/polarssl/md.h
index e94f9be..ae38aad 100644
--- a/include/polarssl/md.h
+++ b/include/polarssl/md.h
@@ -28,13 +28,10 @@
 
 #include <string.h>
 
-#if defined(_MSC_VER) && !defined(inline)
-#define inline _inline
-#else
-#if defined(__ARMCC_VERSION) && !defined(inline)
+#if ( defined(__ARMCC_VERSION) || defined(_MSC_VER) ) && \
+    !defined(inline) && !defined(__cplusplus)
 #define inline __inline
-#endif /* __ARMCC_VERSION */
-#endif /*_MSC_VER */
+#endif
 
 #define POLARSSL_ERR_MD_FEATURE_UNAVAILABLE                -0x5080  /**< The selected feature is not available. */
 #define POLARSSL_ERR_MD_BAD_INPUT_DATA                     -0x5100  /**< Bad input parameters to function. */