at: fix compile breakage when flex isn't on the target

Add a patch that makes sure config.h gets included in a file that uses
some of the NEEDS_* macros. The yywrap() function was missing when
flex was not available on the target, even though this configuration
is supposed to be supported by at (and handled by NEEDS_YYWRAP).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>