proxy: use flb_compat.h to include Windows-specific headers
proxy: use flb_compat.h to include Windows-specific headers

The parent commit adds an ifdef helper for dlfcn.h to flb_compat.h

This patch switches flb_plugin_proxy.c to rely on the functionality
instead of doing manual ifdefs.

Signed-off-by: Fujimoto Seiji <ef879192bb1e41d519c82cf76440dc8e9a530806@clear-code.com>

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