tools/classpath-config: Fix unsignedness issue

Spotted by GCC:

  LINK     tools/classpath-config
tools/classpath-config.c: In function ‘int main(int, char**)’:
tools/classpath-config.c:44:61: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Pekka Enberg <penberg@kernel.org>