fixup??? mem_pool: add GIT_TRACE_MEMPOOL support

The `#include "trace.h"` that was implied until 15db4e7 (treewide:
remove unnecessary cache.h includes in source files, 2023-02-24) now
needs to be made explicit.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

mem-pool.c

@@ -4,6 +4,7 @@
 
 #include "git-compat-util.h"
 #include "mem-pool.h"
+#include "trace.h"
 
 static struct trace_key trace_mem_pool = TRACE_KEY_INIT(MEMPOOL);
 #define BLOCK_GROWTH_SIZE (1024 * 1024 - sizeof(struct mp_block))