add strings.h import
    
    Fixes:
    xmlif/xmlif.l:162:13: warning: implicit declaration of function ‘strncasecmp’; did you mean ‘strncmp’? [-Wimplicit-function-declaration]
      162 |         if (strncasecmp(selections[i], attr, eqoffset) == 0)
          |             ^~~~~~~~~~~
    
    Signed-off-by: Thomas Kuehne <thomas@kuehne.cn>