pgrep: fix -x option

pgrep: fix -x option

Because when -x is used (exact match), then we cannot compile the
regular expression with REG_NOSUB. The manual page regcomp(3) states
in section "Byte offsets":

    Unless REG_NOSUB was set for the compilation of the pattern
    buffer, it is possible to obtain substring match addressing
    information.

The problem was detected on an ARM system with glibc 2.16.
Signed-off-by: Bernhard Walle <bernhard@bwalle.de>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>