[libpng15] Bump version to 1.5.15beta07

pngtest.c

@@ -1949,4 +1949,4 @@ main(void)
 #endif
 
 /* Generate a compiler error if there is an old png.h in the search path. */
-typedef png_libpng_version_1_5_15beta06 Your_png_h_is_not_version_1_5_15beta06;
+typedef png_libpng_version_1_5_15beta07 Your_png_h_is_not_version_1_5_15beta07;