storage.c: Fix compiler error due to missing writev() declaration

storage.c:90:7: error: implicit declaration of function ‘writev’; did you mean ‘write’? [-Werror=implicit-function-declaration]
   90 |  rc = writev(g_out_fd, iov, ARRAY_SIZE(iov));
      |       ^~~~~~
      |       write

Change-Id: If98737199f5a6e8fb37a4fd6403ee973dcf70612

src/storage.c

@@ -4,6 +4,7 @@
 #include <errno.h>
 #include <unistd.h>
 #include <sys/stat.h>
+#include <sys/uio.h>
 
 #include <osmocom/core/msgb.h>
 #include <osmocom/core/talloc.h>