[offload][lit] Fix compilation of two offload tests (#169399)

These are C tests, not C++, so no function parameters means unspecified
number of parameters, not `void`.

These compile fine on the current tested offload targets because an
error is only
[thrown](https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaDecl.cpp#L10695)
if the calling convention doesn't support variadic arguments, which they
happen to.

When compiling this test for other targets that do not support variadic
arguments, we get an error, which does not seem intentional.

Just add `void` to the parameter list.

---------

Signed-off-by: Nick Sarnie <nick.sarnie@intel.com>

offload/test/offloading/shared_lib_fp_mapping.c

@@ -7,8 +7,8 @@
 
 #include <stdio.h>
 
-extern int func(); // Provided in liba.so, returns 42
-typedef int (*fp_t)();
+extern int func(void); // Provided in liba.so, returns 42
+typedef int (*fp_t)(void);
 
 int main() {
   int x = 0;

offload/test/offloading/static_linking.c

@@ -14,7 +14,7 @@ int foo() {
 }
 #else
 #include <stdio.h>
-int foo();
+int foo(void);
 
 int main() {
   int x = foo();