build: fix RELOC_SECTION security check for bitcoin-util
build: fix RELOC_SECTION security check for bitcoin-util

The binutils we use for gitian builds strips the reloc section from
Windows binaries, which breaks ASLR. As a temporary workaround, export
main(). This is the same workaround as #18702 (bitcoin-cli), and will
fix the currently failing security check:
```bash
+ make -j1 -C src check-security
make: Entering directory '/home/ubuntu/build/bitcoin/distsrc-x86_64-w64-mingw32/src'
Checking binary security...
bitcoin-util.exe: failed RELOC_SECTION
make: *** [check-security] Error 1
```

Relevant upstream issue:
https://sourceware.org/bugzilla/show_bug.cgi?id=19011

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in/bitcoin,EthanHeilman/bitcoin,andreaskern/bitcoin,qtumproject/qtum,n1bor/bitcoin