libssh2: do not rely on _WINDLL or DLL_EXPORT defines

[valgur]

Static libssh2 Windows packages are prone to failing in consuming packages due to libssh2 relying on unnamespaced _WINDLL and DLL_EXPORT shared/static defines.

Should fix the unresolved external symbol __imp_libssh2_init errors in test_package for #18810.

Similar to #23680.

[conan-center-bot]

Conan v1 pipeline ✔️

All green in build 1 (3d447aea8933491d61e91b9ed0e49d2767dd974a):

• libssh2/1.9.0:
  All packages built successfully! (All logs)

• libssh2/1.8.2:
  All packages built successfully! (All logs)

• libssh2/1.11.0:
  All packages built successfully! (All logs)

• libssh2/1.10.0:
  All packages built successfully! (All logs)

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Conan v2 pipeline ✔️

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All green in build 1 (3d447aea8933491d61e91b9ed0e49d2767dd974a):

• libssh2/1.8.2:
  All packages built successfully! (All logs)

• libssh2/1.11.0:
  All packages built successfully! (All logs)

• libssh2/1.10.0:
  All packages built successfully! (All logs)

• libssh2/1.9.0:
  All packages built successfully! (All logs)

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