Updated declaration to match definition

[radarhere]

Pointed out in #5526. Currently, for a,

Pillow/src/libImaging/Imaging.h

Lines 364 to 375 in 61b9065

	extern Imaging
	ImagingTransform(
	Imaging imOut,
	Imaging imIn,
	int method,
	int x0,
	int y0,
	int x1,
	int y1,
	double *a,
	int filter,
	int fill);

does not match

Pillow/src/libImaging/Geometry.c

Lines 1126 to 1137 in 61b9065

	Imaging
	ImagingTransform(
	Imaging imOut,
	Imaging imIn,
	int method,
	int x0,
	int y0,
	int x1,
	int y1,
	double a[8],
	int filterid,
	int fill) {

This PR fixes that, updating the declaration.