Add missing arm64-neon function declaration.

I'm trying to get to where we can turn on -Werror=implicit-function-declaration
for LP64, so we can avoid bugs where we drop the top half of a pointer because
the compiler's assuming 'int'.

Change-Id: Ia3508c7807e6f743e97af11b4b0b9633882f80af
diff --git a/simd/jsimd.h b/simd/jsimd.h
index 3d4751f..109a4c3 100644
--- a/simd/jsimd.h
+++ b/simd/jsimd.h
@@ -385,6 +385,10 @@
         JPP((JDIMENSION out_width,
              JSAMPIMAGE input_buf, JDIMENSION input_row,
              JSAMPARRAY output_buf, int num_rows));
+EXTERN(void) jsimd_ycc_rgb565_convert_neon
+        JPP((JDIMENSION out_width,
+             JSAMPIMAGE input_buf, JDIMENSION input_row,
+             JSAMPARRAY output_buf, int num_rows));
 
 /* SIMD Downsample */
 EXTERN(void) jsimd_h2v2_downsample_mmx