libc: fix sparse warning for __assert()

Sparse warns this:

lib/libc/assert.c:29:6: error: symbol '__assert' redeclared with different type (originally declared at include/lib/libc/assert.h:36) - different modifiers

Add __dead2 to match the header declaration and C definition.

I also changed '__dead2 void' to 'void __dead2' for the consistency
with other parts.

Change-Id: Iefa4f0e787c24fa7e7e499d2e7baf54d4deb49ef
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/include/lib/libc/assert.h b/include/lib/libc/assert.h
index d04f9dc..486bbc2 100644
--- a/include/lib/libc/assert.h
+++ b/include/lib/libc/assert.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2018-2019, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -30,12 +30,12 @@
 #endif /* ENABLE_ASSERTIONS */
 
 #if PLAT_LOG_LEVEL_ASSERT >= LOG_LEVEL_VERBOSE
-__dead2 void __assert(const char *file, unsigned int line,
+void __dead2 __assert(const char *file, unsigned int line,
 		      const char *assertion);
 #elif PLAT_LOG_LEVEL_ASSERT >= LOG_LEVEL_INFO
-__dead2 void __assert(const char *file, unsigned int line);
+void __dead2 __assert(const char *file, unsigned int line);
 #else
-__dead2 void __assert(void);
+void __dead2 __assert(void);
 #endif
 
 #endif /* ASSERT_H */