Replace __{BEGIN,END}_DECLS with #ifdef __cplusplus

Fixes the build with musl libc.

Cherry-picked-from: 890f911e17
Link: <https://github.com/shadow-maint/shadow/pull/789>
Reviewed-by: Michael Vetter <jubalh@iodoru.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/readpassphrase.h

@@ -36,8 +36,12 @@
 #endif
 #include <sys/types.h>
 
-__BEGIN_DECLS
+#ifdef __cplusplus
+extern "C" {
+#endif
 char * readpassphrase(const char *, char *, size_t, int);
-__END_DECLS
+#ifdef __cplusplus
+}
+#endif
 
 #endif /* !LIBBSD_READPASSPHRASE_H */