treewide: Add missing string.h includes

Add string.h to files that use string functions like strdup, strcmp,
strcpy, etc. These are implicitly available through the malloc.h header
but that will soon change.

For bouncebuf, take this opportunity to sort the headers correctly.

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

boot/expo_build_cb.c

@@ -14,6 +14,7 @@
 #include <expo.h>
 #include <log.h>
 #include <malloc.h>
+#include <string.h>
 #include <vsprintf.h>
 #include <asm/cb_sysinfo.h>