Reflect on some incompatible changes in GCC 10.

Starting with GCC10 and LTO enabled, current symbols
versioning hack does not work anymore. This patch
reflects on that and should be compatible with older
compilers that does not support __attribute__((symver))
yet.

Inspired by following code:
https://github.com/linux-rdma/rdma-core/blob/master/util/symver.h

lib/Makemodule.am

@@ -43,6 +43,7 @@ libcryptsetup_la_SOURCES = \
 	lib/nls.h			\
 	lib/libcryptsetup.h		\
 	lib/libcryptsetup_macros.h	\
+	lib/libcryptsetup_symver.h	\
 	lib/utils.c			\
 	lib/utils_benchmark.c		\
 	lib/utils_crypt.c		\