Revert "pppd: Include time.h before using time_t"

Revert "pppd: Include time.h before using time_t"

This reverts commit b6cd5586f72852f56428749202779af596353b9e
because it break compilation on Solaris with the following error:

/opt/SUNWspro/bin/cc -D_KERNEL -DSVR4 -DSOL2 -DPRIOQ -DDEBUG
-I../include -O -Xa -xO2 -xspace -W0,-Lt -c ppp.c
"ppp.c", line 113: identifier redeclared: time
        current : long
        previous: function(pointer to long) returning long :
"/usr/include/iso/time_iso.h", line 91
cc: acomp failed for ppp.c

The include/net/ppp_defs.h header is used in the Solaris kernel
driver and hence can't include userland headers.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>