Declare gdImageGetInterpolationMethod() for bundled GD (GH-15329)

When this function has been added to our bundled GD[1], it had been
overlooked to also declare it in gd.h, like it's done in libgd.  While
MSVC doesn't have any issues with this, clang reports an error.

[1] <03bd433>

Author: cmb69

ext/gd/libgd/gd.h

@@ -878,6 +878,7 @@ gdImagePtr gdImageCropAuto(gdImagePtr im, const unsigned int mode);
 gdImagePtr gdImageCropThreshold(gdImagePtr im, const unsigned int color, const float threshold);
 
 int gdImageSetInterpolationMethod(gdImagePtr im, gdInterpolationMethod id);
+gdInterpolationMethod gdImageGetInterpolationMethod(gdImagePtr im);
 
 gdImagePtr gdImageScaleBilinear(gdImagePtr im, const unsigned int new_width, const unsigned int new_height);
 gdImagePtr gdImageScaleBicubic(gdImagePtr src_img, const unsigned int new_width, const unsigned int new_height);