Enable -Werror in CI

And fixes some warnings.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/kernel.h

@@ -13,8 +13,7 @@ typedef struct conv {
 
 /// Calculate the sum of a rectangle part of the convolution kernel
 /// the rectangle is defined by top left (x, y), and a size (width x height)
-double attr_const attr_pure sum_kernel(const conv *map, int x, int y, int width,
-                                       int height);
+double attr_const sum_kernel(const conv *map, int x, int y, int width, int height);
 
 /// Create a kernel with gaussian distribution of radius r
 conv *gaussian_kernel(double r);