Cleanup header files

- All now include maxscale/cdefs.h as the very first file.
- MXS_[BEGIN|END]_DECLS added to all C-headers.
  Strictly speaking not necessary for private headers, but
  does not hurt either.
- Include guards moved to the very top of the file.
- #pragma once added.

include/maxscale/pcre2.h

@@ -1,3 +1,4 @@
+#pragma once
 #ifndef _MAXSCALE_PCRE2_H
 #define _MAXSCALE_PCRE2_H
 /*
@@ -14,12 +15,16 @@
  *
  */
 
+#include <maxscale/cdefs.h>
+
 #ifndef PCRE2_CODE_UNIT_WIDTH
 #define PCRE2_CODE_UNIT_WIDTH 8
 #endif
 
 #include <pcre2.h>
 
+MXS_BEGIN_DECLS
+
 /**
  * @file pcre2.h - Utility functions for regular expression matching
  * with the bundled PCRE2 library.
@@ -44,4 +49,6 @@ mxs_pcre2_result_t mxs_pcre2_substitute(pcre2_code *re, const char *subject,
 mxs_pcre2_result_t mxs_pcre2_simple_match(const char* pattern, const char* subject,
                                           int options, int* error);
 
+MXS_END_DECLS
+
 #endif