cpu/msp430-common: Add stdlib.h header to ensure malloc is defined.

Fixes warnings of the kind:

/data/riotbuild/riotbase/sys/bloom/bloom.c: In function 'bloom_new':
/data/riotbuild/riotbase/sys/bloom/bloom.c:39:5: error: implicit declaration of function 'calloc' [-Werror=implicit-function-declaration]
/data/riotbuild/riotbase/sys/bloom/bloom.c:39:22: error: incompatible implicit declaration of built-in function 'calloc' [-Werror]

cpu/msp430-common/include/stdlib.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (C) 2015 Eistec AB
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser General
+ * Public License v2.1. See the file LICENSE in the top level directory for more
+ * details.
+ */
+
+/**
+ * @ingroup     msp430
+ * @{
+ *
+ * @file
+ * @brief       stdlib.h wrapper for MSP430
+ * @see http://www.cplusplus.com/reference/cstdlib/
+ *
+ * @author      Joakim Gebart <joakim.gebart@eistec.se
+ */
+
+#ifndef RIOT_MSP430_STDLIB_H_
+#define RIOT_MSP430_STDLIB_H_
+
+/*
+ * The MSP430 toolchain does not provide malloc, free, calloc etc. in stdlib.h.
+ * As a workaround, we include the system stdlib.h here and then add malloc.
+ */
+
+#include_next <stdlib.h>
+
+#include <malloc.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* RIOT_MSP430_STDLIB_H_ */