conan: Do not bring Expat when xmp=True.

Since the external XMP SDK is already adding the expat sources to the XMPCore library, we do not
need the Expat dependency for Exiv2.

conanfile.py

@@ -17,7 +17,6 @@ class Exiv2Conan(ConanFile):
         self.options['gtest'].shared = True
 
     def requirements(self):
-        self.requires('Expat/2.2.5@pix4d/stable')
         self.requires('zlib/1.2.11@conan/stable')
         self.requires('libcurl/7.60.0@bincrafters/stable')
 
@@ -26,6 +25,8 @@ class Exiv2Conan(ConanFile):
 
         if self.options.xmp:
             self.requires('XmpSdk/2016.7@piponazo/stable') # from conan-piponazo
+        else:
+            self.requires('Expat/2.2.5@pix4d/stable')
 
     def imports(self):
         self.copy('*.dll', dst='conanDlls', src='bin')