[CMAKE] Fix a -Wpointer-to-int-cast when searching Jemalloc

Summary:
This would trigger failures when `-Werror` is enabled, such as:
https://build.bitcoinabc.org/viewLog.html?tab=buildLog&logTab=tree&filter=debug&expand=all&buildId=66573&guest=1#footer

Test Plan:
  cmake -GNinja .. \
    -DUSE_JEMALLOC_EXPERIMENTAL=ON \
    -DCMAKE_C_FLAGS=-Werror
This will fail before this patch and succeed after.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6343

cmake/utils/CheckJemallocBuilds.c

@@ -1,6 +1,7 @@
 #include <jemalloc.h>
 
 int main(int argc, char** argv) {
+    void *volatile dummyPtr = malloc(42);
     (void)argv;
-    return argc + (int)(malloc(42));
+    return argc;
 }