util/StaticFifoBuffer: add `constexpr`

src/util/StaticFifoBuffer.hxx

@@ -78,11 +78,11 @@ public:
 		head = tail = 0;
 	}
 
-	bool empty() const noexcept {
+	constexpr bool empty() const noexcept {
 		return head == tail;
 	}
 
-	bool IsFull() const noexcept {
+	constexpr bool IsFull() const noexcept {
 		return head == 0 && tail == size;
 	}
 
@@ -115,7 +115,7 @@ public:
 	 * Return a buffer range which may be read.  The buffer pointer is
 	 * writable, to allow modifications while parsing.
 	 */
-	Range Read() noexcept {
+	constexpr Range Read() noexcept {
 		return Range(data + head, tail - head);
 	}