avoid any potential for multiple-defined symbols for M_PI

gfx/cairo/libpixman/src/fbcompose.c

@@ -37,12 +37,12 @@
 #define _USE_MATH_DEFINES
 #endif
 
+#include <math.h>
+
 #ifndef M_PI
 #define M_PI 3.14159265358979323846
 #endif
 
-#include <math.h>
-
 // #define PIXMAN_CONVOLUTION
 // #define PIXMAN_INDEXED_FORMATS