sdbus++: add common header file

There are some items generated under server and client which are
overlapping and some currently in server that will also be needed
for async bindings.  Create a common header file that they can
all include in order to reduce the content generated across all these
types.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Id7af8542613503dbd745158c89892ca252eadf5a
diff --git a/example/gen/meson.build b/example/gen/meson.build
index a7da0a7..23394ac 100644
--- a/example/gen/meson.build
+++ b/example/gen/meson.build
@@ -5,10 +5,10 @@
     check: true,
 ).stdout().strip().split('\n')[0]
 
-if sdbuspp_gen_meson_ver != 'sdbus++-gen-meson version 5'
+if sdbuspp_gen_meson_ver != 'sdbus++-gen-meson version 6'
     warning('Generated meson files from wrong version of sdbus++-gen-meson.')
     warning(
-        'Expected "sdbus++-gen-meson version 5", got:',
+        'Expected "sdbus++-gen-meson version 6", got:',
         sdbuspp_gen_meson_ver
     )
 endif
diff --git a/example/gen/net/poettering/Calculator/meson.build b/example/gen/net/poettering/Calculator/meson.build
index 6b469e6..e906ab2 100644
--- a/example/gen/net/poettering/Calculator/meson.build
+++ b/example/gen/net/poettering/Calculator/meson.build
@@ -2,7 +2,7 @@
 generated_sources += custom_target(
     'net/poettering/Calculator__cpp'.underscorify(),
     input: [ '../../../../yaml/net/poettering/Calculator.interface.yaml',  ],
-    output: [ 'error.cpp', 'error.hpp', 'server.cpp', 'server.hpp', 'client.hpp',  ],
+    output: [ 'error.cpp', 'error.hpp', 'common.hpp', 'server.cpp', 'server.hpp', 'client.hpp',  ],
     depend_files: sdbusplusplus_depfiles,
     command: [
         sdbuspp_gen_meson_prog, '--command', 'cpp',