Platform: Fix uart_stdout.h include

The call to stdio_init() has moved from tfm_core.c to tfm_platform.c,
so moves the corresponding include.

Change-Id: I72bb1ab7a22a33e5cc108756eccf658d480d7384
Signed-off-by: Jamie Fox <jamie.fox@arm.com>
diff --git a/platform/ext/common/tfm_platform.c b/platform/ext/common/tfm_platform.c
index 2eeddf5..48e56f1 100644
--- a/platform/ext/common/tfm_platform.c
+++ b/platform/ext/common/tfm_platform.c
@@ -7,6 +7,7 @@
 
 #include "target_cfg.h"
 #include "platform/include/tfm_spm_hal.h"
+#include "uart_stdout.h"
 
 /* platform-specific hw initialization */
 __WEAK enum tfm_plat_err_t tfm_spm_hal_post_init_platform(void)