Speculatively fix build bots

This should address build failures found in:

https://lab.llvm.org/buildbot/#/builders/217/builds/3610
https://lab.llvm.org/buildbot/#/builders/215/builds/4609
https://lab.llvm.org/buildbot/#/builders/68/builds/31012

bolt/test/Inputs/asm_main.c

@@ -1,3 +1,5 @@
+void foo(void);
+
 int main() {
   foo();
   return 0;

cross-project-tests/debuginfo-tests/dexter-tests/dbg-arg.c

@@ -18,6 +18,7 @@ struct _mtx
   } mtxi;
 };
 
+int bar(int, int);
 
 int foobar(struct _mtx *mutex) {
   int r = 1;

lldb/test/API/functionalities/thread/exit_during_expression/main.c

@@ -5,6 +5,8 @@
 
 static unsigned int g_timeout = 100000;
 
+extern int usleep(unsigned int);
+
 int function_to_call() {
 
   errno = 0;