[tools] Fix -Wstrict-prototypes in llvm-c-test.h (NFC)

/llvm-project/llvm/tools/llvm-c-test/llvm-c-test.h:39:24:
error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
int llvm_test_dibuilder();
                       ^
                        void
1 error generated.

Author: DamonFool

llvm/tools/llvm-c-test/debuginfo.c

@@ -29,7 +29,7 @@ declare_objc_class(LLVMDIBuilderRef DIB, LLVMMetadataRef File) {
   return Decl;
 }
 
-int llvm_test_dibuilder() {
+int llvm_test_dibuilder(void) {
   const char *Filename = "debuginfo.c";
   LLVMModuleRef M = LLVMModuleCreateWithName(Filename);
 

llvm/tools/llvm-c-test/llvm-c-test.h

@@ -36,7 +36,7 @@ int llvm_calc(void);
 int llvm_disassemble(void);
 
 // debuginfo.c
-int llvm_test_dibuilder();
+int llvm_test_dibuilder(void);
 int llvm_get_di_tag(void);
 int llvm_di_type_get_name(void);