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/library/ecp.c b/library/ecp.c
index 858540b..64a47ab 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -62,7 +62,8 @@
 #define mbedtls_free       free
 #endif
 
-#if ( defined(__ARMCC_VERSION) || defined(_MSC_VER) ) && !defined(inline)
+#if ( defined(__ARMCC_VERSION) || defined(_MSC_VER) ) && \
+    !defined(inline) && !defined(__cplusplus)
 #define inline __inline
 #endif