[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