fix(st): add missing header include

This issue is triggered when enabling -Wmissing-prototypes:
plat/st/common/bl2_io_storage.c:114:5: warning: no previous prototype
 for 'open_fip' [-Wmissing-prototypes]
  114 | int open_fip(const uintptr_t spec)
      |     ^~~~~~~~
plat/st/common/bl2_io_storage.c:119:5: warning: no previous prototype
 for 'open_storage' [-Wmissing-prototypes]
  119 | int open_storage(const uintptr_t spec)
      |     ^~~~~~~~~~~~

Add missing stm32mp_io_storage.h header include, where those functions
prototypes are defined.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I2af69fadfc4780553f41b338cd93b731210672a6
diff --git a/plat/st/common/bl2_io_storage.c b/plat/st/common/bl2_io_storage.c
index 5cc3390..da4c28d 100644
--- a/plat/st/common/bl2_io_storage.c
+++ b/plat/st/common/bl2_io_storage.c
@@ -38,6 +38,7 @@
 #include <platform_def.h>
 #include <stm32cubeprogrammer.h>
 #include <stm32mp_fconf_getter.h>
+#include <stm32mp_io_storage.h>
 #include <usb_dfu.h>
 
 /* IO devices */