pm-utils: Do not require GNU grep at runtime

This was added to fix bug reported here [1]

back then busybox grep applet did not implement -x option
and it would fail as reported in the bug, in due course
busybox now has implemented -x option [2] and the expression

using grep -x in /usr/lib/pm-utils/functions:219 works fine

[1] https://bugzilla.yoctoproject.org/show_bug.cgi?id=1887
[2] https://git.busybox.net/busybox/commit/?id=cd09e81520b7917adebcffd7c361671f913325eb



Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>