fix compiler warning about ambiguous else

i686-w64-mingw32-gcc -c -pipe -O2 -DWINVER=0x501   -mwindows -Wall -Ipcre  parsers.c -o parsers.o
In file included from parsers.c:90:0:
strptime.h: In function 'strptime_internal':
strptime.h:538:7: warning: suggest explicit braces to avoid ambiguous 'else' [-Wparentheses]
    if (!match_string (HERE_AM_STR, rp))
       ^