Fix test cases/common/135 on Cygwin

I don't really know how to explain this briefly...

If you don't decorate this with dllimport, then a Cygwin runtime relocation
is used (a so called 'pseudo-reloc').  As the relocation offset is only 32
bits, this can fail on x86_64 if the DLL happens to be loaded more than 2GB
away from the reference.

If you decorate with dllimport, then access is indirected via a pointer,
imp_square_unsigned, which is fixed up by the loader.

test cases/common/135 generated assembly/main.c

@@ -1,5 +1,8 @@
 #include <stdio.h>
 
+#if defined(_WIN32) || defined(__CYGWIN__)
+ __declspec(dllimport)
+#endif
 unsigned square_unsigned (unsigned a);
 
 int