package/darkhttpd: fix build with gcc 4.8

package/darkhttpd: fix build with gcc 4.8

Fix the following build failure which is raised since bump to version
0.13 in commit 88705a8c248e566f2c9ca71a8ee5353b33e368c2:

darkhttpd.c:966:5: error: 'for' loop initial declarations are only allowed in C99 mode
     for (int i = 0, j = 0; i < input_length;) {
     ^

Fixes:
 - http://autobuild.buildroot.org/results/ffc/ffcb627c44d2b7202d641129b67af2dc7865c618/build-end.log

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>