Add inline definition for Microsoft Visual Studio

Summary:
The inline keyword is available only in C++. The __inline and __forceinline keywords are available in both C and C++. For compatibility with previous versions, _inline is a synonym for __inline.

https://msdn.microsoft.com/en-us/library/z8y1yy88(v=vs.120).aspx
Closes https://github.com/facebook/css-layout/pull/239

Reviewed By: astreet

Differential Revision: D4138941

Pulled By: emilsjolander

fbshipit-source-id: cb59dc91ef285e5378036c4912217fd4ec8d9f79

CSSLayout/CSSLayout.c

@@ -16,6 +16,10 @@
 #include <float.h>
 #define isnan _isnan
 
+#ifndef __cplusplus
+#define inline __inline
+#endif
+
 /* define fmaxf if < VC12 */
 #if _MSC_VER < 1800
 __forceinline const float fmaxf(const float a, const float b) {