gn: Don't define WINVER twice for target usrsctp.
gn: Don't define WINVER twice for target usrsctp.

Without this, both -DWINVER=0x0502 (from this file) and -DWINVER=0x0603 (from
build/config/win/BUILD.gn) are added as compiler flags for this target.  This
is confusing, and clang happens to warn about it too.  usrsctp.gyp is already
careful to not define this twice, make gn match the gyp build.

BUG=491209

Review URL: https://codereview.chromium.org/1160203004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#332776}

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