fix OpenBSD compile issue

Linux seems the only system AFAICS to have alloca.h.  All the BSDs have
the declaration in stdlib.h

src/lib/sw_engine/tvgSwRaster.cpp

@@ -22,10 +22,10 @@
 
 #ifdef _WIN32
     #include <malloc.h>
-#elif defined(__FreeBSD__)
-    #include <stdlib.h>
-#else
+#elif defined(__linux__)
     #include <alloca.h>
+#else
+    #include <stdlib.h>
 #endif
 
 #include "tvgMath.h"

src/loaders/svg/tvgXmlParser.cpp

@@ -26,10 +26,10 @@
 
 #ifdef _WIN32
     #include <malloc.h>
-#elif defined(__FreeBSD__)
-    #include <stdlib.h>
-#else
+#elif defined(__linux__)
     #include <alloca.h>
+#else
+    #include <stdlib.h>
 #endif
 
 #include "tvgXmlParser.h"

src/loaders/tvg/tvgTvgBinInterpreter.cpp

@@ -23,10 +23,10 @@
 
 #ifdef _WIN32
     #include <malloc.h>
-#elif defined(__FreeBSD__)
-    #include <stdlib.h>
-#else
+#elif defined(__linux__)
     #include <alloca.h>
+#else
+    #include <stdlib.h>
 #endif
 
 #include "tvgTvgCommon.h"

src/savers/tvg/tvgTvgSaver.cpp

@@ -28,10 +28,10 @@
 
 #ifdef _WIN32
     #include <malloc.h>
-#elif defined(__FreeBSD__)
-    #include <stdlib.h>
-#else
+#elif defined(__linux__)
     #include <alloca.h>
+#else
+    #include <stdlib.h>
 #endif
 
 static FILE* _fopen(const char* filename, const char* mode)