Export IsUnderPostmaster for pg_stat_statements on win32

Hi,

I compiled pg_stat_statements on mingw, and got the following error.
PGDLLIMPORT for IsUnderPostmaster seems to be needed on win32.

----
Info: resolving _IsUnderPostmaster by linking to __imp__IsUnderPostmaster (auto-import)
fu000001.o:(.idata$2+0xc): undefined reference to `libpostgres_a_iname'
nmth000000.o:(.idata$4+0x0): undefined reference to `_nm__IsUnderPostmaster'
----

Patch attached.

Index: src/include/miscadmin.h
===================================================================
--- src/include/miscadmin.h	(HEAD)
+++ src/include/miscadmin.h	(fixed)
@@ -123,7 +123,7 @@
  */
 extern pid_t PostmasterPid;
 extern bool IsPostmasterEnvironment;
-extern bool IsUnderPostmaster;
+extern PGDLLIMPORT bool IsUnderPostmaster;

extern bool ExitOnAnyError;

Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center