flowtop: Fix compiler warnings

Fix the following compiler warnings that occur when building with "-W
-Wall -Wextra":

flowtop.c: In function ‘walk_process’:
flowtop.c:417:45: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
flowtop.c: In function ‘presenter_screen_update’:
flowtop.c:937:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
flowtop.c:938:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>