postfix: add support for linux-6

Without this change `postfix` fails to build on `linux-6` as:

    postfix> ATTENTION:
    postfix> ATTENTION: Unknown system type: Linux 6.0.0
    postfix> ATTENTION:

pkgs/servers/mail/postfix/default.nix

@@ -53,6 +53,9 @@ in stdenv.mkDerivation rec {
       url = "https://src.fedoraproject.org/rpms/postfix/raw/2f9d42453e67ebc43f786d98262a249037f80a77/f/postfix-3.6.2-glibc-234-build-fix.patch";
       sha256 = "sha256-xRUL5gaoIt6HagGlhsGwvwrAfYvzMgydsltYMWvl9BI=";
     })
+
+    # linux-6 compatibility
+    ./linux-6.patch
   ];
 
   postPatch = lib.optionalString (stdenv.hostPlatform != stdenv.buildPlatform) ''

pkgs/servers/mail/postfix/linux-6.patch

@@ -0,0 +1,26 @@
+Extracted fix from postfix-3.8-20221006 snapshot:
+    https://github.com/vdukhovni/postfix/commit/b65530350fa4a7eee40946160fd80c3e1e0b63e5
+--- a/makedefs
++++ b/makedefs
+@@ -627,7 +627,8 @@ EOF
+ 		: ${SHLIB_ENV="LD_LIBRARY_PATH=`pwd`/lib"}
+ 		: ${PLUGIN_LD="${CC-gcc} -shared"}
+ 		;;
+- Linux.[345].*)	SYSTYPE=LINUX$RELEASE_MAJOR
++    Linux.[3456].*)
++		SYSTYPE=LINUX$RELEASE_MAJOR
+ 		case "$CCARGS" in
+ 		 *-DNO_DB*) ;;
+ 		 *-DHAS_DB*) ;;
+--- a/src/util/sys_defs.h
++++ b/src/util/sys_defs.h
+@@ -751,7 +751,8 @@ extern int initgroups(const char *, int);
+  /*
+   * LINUX.
+   */
+-#if defined(LINUX2) || defined(LINUX3) || defined(LINUX4) || defined(LINUX5)
++#if defined(LINUX2) || defined(LINUX3) || defined(LINUX4) || defined(LINUX5) \
++	|| defined(LINUX6)
+ #define SUPPORTED
+ #define UINT32_TYPE	unsigned int
+ #define UINT16_TYPE	unsigned short