assert.h: reintroduce headers guard for function declarations.

The POSIX standard requires us to allow assert.h to be included multiple
times with differnt values of NDEBUG. So we can't have a global header
guard on the files. However, we must also make sure that we don't
declare functions multiple times in that case. Re-introduce an header
guard on the part of the file where we declare functions, only.

Fixes lots of warnings when building Netsurf.

headers/posix/assert.h

@@ -3,7 +3,7 @@
  * Distributed under the terms of the MIT License.
  */
 
-/* Include guards are omitted, as assert.h is required
+/* Include guards are only on part of the file, as assert.h is required
    to support being included multiple times.
 
    E.g. the following is required to be valid:
@@ -24,6 +24,9 @@
 #ifndef NDEBUG
 	/* defining NDEBUG disables assert() functionality */
 
+#ifndef _ASSERT_H_
+#define _ASSERT_H_
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -40,6 +43,8 @@ extern void __assert_perror_fail(int error, const char *file,
 }
 #endif
 
+#endif /* !_ASSERT_H_ */
+
 #define assert(assertion) \
 	((assertion) ? (void)0 : __assert_fail(#assertion, __FILE__, __LINE__, __PRETTY_FUNCTION__))