xf86drm: avoid loop initial declarations

xf86drm: avoid loop initial declarations

To avoid the error:

external/libdrm/xf86drm.c: In function 'parse_separate_sysfs_files':
external/libdrm/xf86drm.c:3098:5: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
     for (unsigned i = ignore_revision ? 1 : 0; i < ARRAY_SIZE(attrs); i++) {
     ^
external/libdrm/xf86drm.c:3098:5: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code