liblzma: Avoid compiler warning without creating extra symbol.

When the generic fast crc64 method is used, then we omit
lzma_crc64_table[][]. Similar to
d9166b52cf, we can avoid compiler warnings
with -Wempty-translation-unit (Clang) or -pedantic (GCC) by creating a
never used typedef instead of an extra symbol.

src/liblzma/check/crc64_table.c

@@ -20,8 +20,7 @@
 		|| (defined(__e2k__) && __iset__ >= 6)
 // No table needed but something has to be exported to keep some toolchains
 // happy. Also use a declaration to silence compiler warnings.
-extern const char lzma_crc64_dummy;
-const char lzma_crc64_dummy;
+typedef void lzma_crc64_dummy;
 
 #else
 // Having the declaration here silences clang -Wmissing-variable-declarations.