Include <alloca.h> also on macOS (and osxcross) (#2218)

imgui_draw.cpp

@@ -32,7 +32,7 @@ Index of this file:
 
 #include <stdio.h>      // vsnprintf, sscanf, printf
 #if !defined(alloca)
-#if defined(__GLIBC__) || defined(__sun) || defined(__CYGWIN__)
+#if defined(__GLIBC__) || defined(__sun) || defined(__CYGWIN__) || defined(__APPLE__)
 #include <alloca.h>     // alloca (glibc uses <alloca.h>. Note that Cygwin may have _WIN32 defined, so the order matters here)
 #elif defined(_WIN32)
 #include <malloc.h>     // alloca