viml/profile: revert proftime_T to unsigned type #10521

- reltimestr(): Produce negative value by comparing the unsigned
  proftime_T value to INT64_MAX.

https://github.com/neovim/neovim/issues/10452#issuecomment-511155132
1. The interfaces of nearly all platforms return uint64_t. INT64_MAX is
   only half of that.
2. Low-level interfaces like this typically define that there is no
   fixed starting point. The only guarantees are that it's (a)
   monotonically increasing at a rate that (b) matches real time.

ref 06af88cd72
fix #10452

src/nvim/profile.h

@@ -4,7 +4,7 @@
 #include <stdint.h>
 #include <time.h>
 
-typedef int64_t proftime_T;
+typedef uint64_t proftime_T;
 
 #define TIME_MSG(s) do { \
     if (time_fd != NULL) time_msg(s, NULL); \