fix(corstone1000): include platform header file

Include platform.h file in order to remove following compiler errors,
as some warnings are being treated as errors now.
error: implicit declaration of function
'plat_core_pos_by_mpidr'[-Wimplicit-function-declaration]

Change-Id: Ie223e11e138ec9b0eef7342f450b90b215a49b15
Signed-off-by: Harsimran Singh Tungal <harsimransingh.tungal@arm.com>
diff --git a/plat/arm/board/corstone1000/common/corstone1000_pm.c b/plat/arm/board/corstone1000/common/corstone1000_pm.c
index ee07605..51d696c 100644
--- a/plat/arm/board/corstone1000/common/corstone1000_pm.c
+++ b/plat/arm/board/corstone1000/common/corstone1000_pm.c
@@ -7,6 +7,7 @@
 #include <lib/psci/psci.h>
 #include <plat/arm/common/plat_arm.h>
 #include <platform_def.h>
+#include <plat/common/platform.h>
 /*******************************************************************************
  * Export the platform handlers via plat_arm_psci_pm_ops. The ARM Standard
  * platform layer will take care of registering the handlers with PSCI.