Add new include directory to platform for SLPI

Some SLPI changes are causing build failures on CHRE when not including
the directory in this CL.

Bug: 146991028
Test: Build CHRE
Change-Id: I80e1a2992dbada40fbbfe6ab3e1a968492c293b2
diff --git a/platform/platform.mk b/platform/platform.mk
index 78a778b..2b0f74e 100644
--- a/platform/platform.mk
+++ b/platform/platform.mk
@@ -29,6 +29,7 @@
 SLPI_CFLAGS += -I$(SLPI_PREFIX)/core/api/systemdrivers
 SLPI_CFLAGS += -I$(SLPI_PREFIX)/platform/inc
 SLPI_CFLAGS += -I$(SLPI_PREFIX)/platform/inc/HAP
+SLPI_CFLAGS += -I$(SLPI_PREFIX)/platform/inc/a1std
 SLPI_CFLAGS += -I$(SLPI_PREFIX)/platform/inc/stddef
 SLPI_CFLAGS += -I$(SLPI_PREFIX)/platform/rtld/inc