Mark the CompressionOptions(int) constructor explicit

A single-argument constructor without explicit allows an int to convert
implicitly to CompressionOptions, so an accidental Compress(reader, writer, 2)
compiles without complaint. Nothing in the library relies on the conversion.

snappy.h

diff --git a/snappy.h b/snappy.h
index 1c44868..11fb8ac 100644
--- a/snappy.h
+++ b/snappy.h
@@ -69,7 +69,7 @@
     int level = DefaultCompressionLevel();
 
       constexpr CompressionOptions() = default;
-      constexpr CompressionOptions(int compression_level)
+      constexpr explicit CompressionOptions(int compression_level)
           : level(compression_level) {}
 
     static constexpr int MinCompressionLevel() { return 1; }