fix: add missing PYBIND11_ALWAYS_INLINE macro for v3.0.3 backports

Cherry-picking #5992 without #5887 left this macro undefined and broke builds, so this restores the expected inline helper definition with the smallest possible change.

Made-with: Cursor

include/pybind11/detail/common.h

diff --git a/include/pybind11/detail/common.h b/include/pybind11/detail/common.h
index 2ca1558..7158923 100644
--- a/include/pybind11/detail/common.h
+++ b/include/pybind11/detail/common.h
@@ -167,6 +167,14 @@
 #    define PYBIND11_NOINLINE __attribute__((noinline)) inline
 #endif
 
+#if defined(_MSC_VER)
+#    define PYBIND11_ALWAYS_INLINE __forceinline
+#elif defined(__GNUC__)
+#    define PYBIND11_ALWAYS_INLINE __attribute__((__always_inline__)) inline
+#else
+#    define PYBIND11_ALWAYS_INLINE inline
+#endif
+
 #if defined(__MINGW32__)
 // For unknown reasons all PYBIND11_DEPRECATED member trigger a warning when declared
 // whether it is used or not