ReusableArray: fix build error on GCC7

GCC7 outputs the following error without this change:

    src/util/ReusableArray.hxx:61:35: error: no matching function for call to ‘swap(size_t&, const size_t&)’
       std::swap(capacity, src.capacity);

which can be resolved by just using an rvalue-reference rather than a
const rvalue-reference.

Signed-off-by: Ben Boeckel <mathstuf@gmail.com>

src/util/ReusableArray.hxx

@@ -56,7 +56,7 @@ public:
 		:buffer(std::exchange(src.buffer, nullptr)),
 		 capacity(std::exchange(src.capacity, 0)) {}
 
-	ReusableArray &operator=(const ReusableArray &&src) {
+	ReusableArray &operator=(ReusableArray &&src) {
 		std::swap(buffer, src.buffer);
 		std::swap(capacity, src.capacity);
 		return *this;