util: automake: rework the format_srgb.c rule

util: automake: rework the format_srgb.c rule

A handful of changes/cleanups paving the way to bmake support:
 - Remove optional $(srcdir)/ prefix for files in the prereq list.
 - Drop the space after the AM_V_GEN variable.
 - Using $< in a non-suffix rule is a GNU make idiom.
 - Use $(@D) over $(dir $@). The latter is a POSIX standard.

v2: Cosmetic tweaks in the commit summary.

Cc: 11.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com> (v1)