hostcheckstop: Fix missing header

Fixes the following error:

 host_checkstop_obj.c: In function ‘is_host_booted’:
 host_checkstop_obj.c:46:10: error: implicit declaration of function
 ‘strcmp’ [-Werror=implicit-function-declaration]
      if ((strcmp(system_state, "HOST_BOOTED") == 0) ||

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/hostcheckstop/host_checkstop_obj.c b/hostcheckstop/host_checkstop_obj.c
index 74c940e..c6eab2f 100644
--- a/hostcheckstop/host_checkstop_obj.c
+++ b/hostcheckstop/host_checkstop_obj.c
@@ -1,3 +1,5 @@
+#include <string.h>
+
 #include <openbmc_intf.h>
 #include <openbmc.h>
 #include <gpio.h>