[doxy] fix build when more aggressive -W options are used (#410)

make[2]: Entering directory '/builddir/build/BUILD/kronosnet-1.17/man'
gcc -DHAVE_CONFIG_H -I. -I.. -O3 -ggdb3 -Werror -Wall -Wextra -Wno-unused-parameter -pthread -I/usr/include/libxml2 -c -o doxyxml-doxyxml.o test -f 'doxyxml.c' || echo './'doxyxml.c
doxyxml.c: In function 'allcaps':
doxyxml.c:414:13: error: comparison of integer expressions of different signedness: 'int' and 'size_t' {aka 'long unsigned int'} [-Werror=sign-compare]

414 |  for (i=0; i< strlen(name); i++) {
    |             ^

cc1: all warnings being treated as errors
make[2]: * [Makefile:623: doxyxml-doxyxml.o] Error 1
make[2]: Leaving directory '/builddir/build/BUILD/kronosnet-1.17/man'
make[1]: * [Makefile:567: all-recursive] Error 1

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>