trace2: use size_t alloc,nr_open_regions in tr2tls_thread_ctx

Use "size_t" rather than "int" for the "alloc" and "nr_open_regions"
fields in the "tr2tls_thread_ctx".  These are used by ALLOC_GROW().

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trace2/tr2_tls.h

@@ -11,8 +11,8 @@
 struct tr2tls_thread_ctx {
 	struct strbuf thread_name;
 	uint64_t *array_us_start;
-	int alloc;
+	size_t alloc;
-	int nr_open_regions; /* plays role of "nr" in ALLOC_GROW */
+	size_t nr_open_regions; /* plays role of "nr" in ALLOC_GROW */
 	int thread_id;
 };