Restore SEASTAR_MODULE for module interface file

Headers included in the module purview need explicit 'export' keywords to be
exported. Without SEASTAR_MODULE defined, SEASTAR_MODULE_EXPORT macros expand
to nothing, and declarations are not exported from the module.

The fix:
- Define SEASTAR_MODULE only for seastar.cppm (module interface file)
- This makes SEASTAR_MODULE_EXPORT expand to "export" in the module interface
- Implementation .cc files don't get SEASTAR_MODULE, so no "export" in regular code

This is the correct approach: export keywords are needed and valid only in the
module interface file, not in implementation files.

Co-authored-by: avikivity <[email protected]>

Author: Copilot

src/CMakeLists.txt

@@ -99,6 +99,10 @@ target_compile_definitions (seastar-module
     SEASTAR_SCHEDULING_GROUPS_COUNT=${Seastar_SCHEDULING_GROUPS_COUNT}
   PRIVATE
     ${Seastar_PRIVATE_COMPILE_DEFINITIONS})
+# Define SEASTAR_MODULE only for the module interface file to enable export keywords
+set_source_files_properties (seastar.cppm
+  PROPERTIES
+    COMPILE_DEFINITIONS "SEASTAR_MODULE")
 target_compile_options (seastar-module
   PUBLIC
     -U_FORTIFY_SOURCE)