rb: add cbindgen config for bits_eventfd (eventfd_t type)

bits/eventfd.h defines the eventfd_t 64-bit counter type used by
sys/eventfd.h. Without this config, cbindgen skipped the directory
and the type wasn't generated.

src/header/bits_eventfd/cbindgen.toml

@@ -0,0 +1,14 @@
+# `bits/eventfd.h` — eventfd counter type
+#
+# Type alias for the 64-bit eventfd counter.
+# Used by sys/eventfd.h for read/write operations.
+include_guard = "_RELIBC_BITS_EVENTFD_H"
+language = "C"
+no_includes = true
+cpp_compat = true
+
+[export]
+include = ["eventfd_t"]
+
+[export.rename]
+"eventfd_t" = "eventfd_t"