Remove remaining C++ from kelvin.h, include in hello_world

- Remove the inclusion of <algorithm> from kelvin.h, so that there is no
  C++ remaining and we can include it from C.
- Include the header in hello_world, so that there is a canary for any
  modifications to kelvin.h that would be incompatible.

Change-Id: I1d147395cd85c8f9ff6edae3593d5a68f1fed177

crt/kelvin.h

diff --git a/crt/kelvin.h b/crt/kelvin.h
index cd7be88..9a16617 100644
--- a/crt/kelvin.h
+++ b/crt/kelvin.h
@@ -11,8 +11,6 @@
 #include <stdint.h>
 #include <string.h>
 
-#include <algorithm>
-
 #define __volatile_always__ volatile
 
 // Helper macros for Intrinsics definitions.

examples/hello_world/hello_world.c

diff --git a/examples/hello_world/hello_world.c b/examples/hello_world/hello_world.c
index aa96624..25f114b 100644
--- a/examples/hello_world/hello_world.c
+++ b/examples/hello_world/hello_world.c
@@ -16,6 +16,10 @@
 #include <stddef.h>
 #include <stdint.h>
 
+// This header is included to ensure that it can
+// be referenced from a C file.
+#include "crt/kelvin.h"
+
 typedef struct {
   uint32_t return_code;  // Populated in kelvin_start.S.
   uint32_t output_ptr;