dtrace tests: fix prototypes for gcc build

- quiet -Wstrict-prototypes
 - provide prototypes for weak aliases

Reviewed by:	markj
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D28036

cddl/contrib/opensolaris/cmd/dtrace/test/tst/common/java_api/tst.FunctionLookup.c

@@ -41,7 +41,7 @@ f2(f func, int i)
 }
 
 int
-main()
+main(void)
 {
 	f2(f1, 3);
 

cddl/contrib/opensolaris/cmd/dtrace/test/tst/common/java_api/tst.ProbeData.c

@@ -66,7 +66,7 @@ fN2(f func, char *a, int i __unused)
 }
 
 int
-main()
+main(void)
 {
 	/*
 	 * Avoid length of 1, 2, 4, or 8 bytes so DTrace will treat the data as

cddl/contrib/opensolaris/cmd/dtrace/test/tst/common/pid/tst.weak1.c

@@ -33,8 +33,7 @@
  * leading underscores.
  */
 
-#pragma weak _go = go
-
+extern int _go(int);
 int go(int);
 
 int
@@ -57,3 +56,5 @@ main(void)
 	for (;;)
 		getpid();
 }
+
+#pragma weak _go = go

cddl/contrib/opensolaris/cmd/dtrace/test/tst/common/pid/tst.weak2.c

@@ -33,14 +33,14 @@
  * leading underscores.
  */
 
+extern int _go(int);
+
 static int __unused
 go(int a)
 {
 	return (a + 1);
 }
 
-#pragma weak _go = go
-
 static void
 handle(int sig __unused)
 {
@@ -55,3 +55,5 @@ main(void)
 	for (;;)
 		getpid();
 }
+
+#pragma weak _go = go