libdwfl: Add some extra space to buffer to read kernel image header

GCC 14 notices we play some tricks with the array into which we try
to read the kernel image header.

image-header.c: In function ‘__libdw_image_header’:
image-header.c:77:18: error: array subscript -496 is outside array bounds of ‘char[96]’ [-Werror=array-bounds=]
   77 |           header = header_buffer - H_START;
      |                  ^
image-header.c:67:12: note: at offset -496 into object ‘header_buffer’ of size 96
   67 |       char header_buffer[H_READ_SIZE];
      |            ^~~~~~~~~~~~~

GCC is correct. The new header pointer is before the actually buffer we
want to read from. Later in the code we "correct" the address again by
adding the "offset" off the elements we want to read. Such pointer
arithmetic is technically invalid. Make it valid by making the buffer
a little bigger, so all pointer arithmetic stays inside the header_buffer.
This does waste 496 bytes on the stack at the front of the buffer that
is never used.

	* libdwfl/image-header.c (__libdw_image_header): Add H_START
	to header_buffer size and return

Signed-off-by: Mark Wielaard <[email protected]>

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