block/qapi: Fix Sparse warning

Sparse reports this warning:

block/qapi.c:417:47: warning:
 too long initializer-string for array of char(no space for nul char)

Replacing the string by an array of characters fixes this warning.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

block/qapi.c

@@ -414,7 +414,7 @@ BlockStatsList *qmp_query_blockstats(bool has_query_nodes,
 
 static char *get_human_readable_size(char *buf, int buf_size, int64_t size)
 {
-    static const char suffixes[NB_SUFFIXES] = "KMGT";
+    static const char suffixes[NB_SUFFIXES] = {'K', 'M', 'G', 'T'};
     int64_t base;
     int i;