Don't mark the compatibility header module map as requiring Objective-C

The module map generated for the compatibility header was marked as
`requires objc`. This triggers an error when importing it from a C source
file.

This declaration is superfluous, the compatibility header is already
printed in a way where the Objective-C code is protected behind a
language check. C clients can safely import the current compatibility
header even if it may not see any content.

Let's lift this restriction. It isn't currently necessary and we're
adding C content to the compatibility header with the official support
for `@cdecl` that is independent of Objective-C.

Author: xymus

Sources/Build/BuildDescription/SwiftModuleBuildDescription.swift

@@ -898,7 +898,6 @@ public final class SwiftModuleBuildDescription {
             #"""
             module \#(self.target.c99name) {
                 header "\#(self.objCompatibilityHeaderPath.pathString)"
-                requires objc
             }
 
             """#.utf8