clang-tidy: make several member functions const

Found with readability-make-member-function-const

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/actions.cpp

@@ -82,7 +82,7 @@ namespace {
         //! Read the timestamp from a broken-down time in buffer \em tm.
         int read(struct tm* tm);
         //! Set the timestamp of a file
-        int touch(const std::string& path);
+        int touch(const std::string& path) const;
 
     private:
         time_t actime_{0};
@@ -1790,7 +1790,7 @@ namespace {
         return rc;
     }
 
-    int Timestamp::touch(const std::string& path)
+    int Timestamp::touch(const std::string& path) const
     {
         if (0 == actime_) return 1;
         struct utimbuf buf;

src/ini.cpp

@@ -233,7 +233,7 @@ INIReader::INIReader(const std::string &filename)
     _error = ini_parse(filename.c_str(), ValueHandler, this);
 }
 
-int INIReader::ParseError()
+int INIReader::ParseError() const
 {
     return _error;
 }

src/pgfimage.cpp

@@ -263,7 +263,7 @@ namespace Exiv2 {
         return b;
     } // PgfImage::readPgfMagicNumber
 
-    uint32_t PgfImage::readPgfHeaderSize(BasicIo& iIo)
+    uint32_t PgfImage::readPgfHeaderSize(BasicIo& iIo) const
     {
         DataBuf buffer(4);
         long bufRead = iIo.read(buffer.pData_, buffer.size_);
@@ -280,7 +280,7 @@ namespace Exiv2 {
         return headerSize;
     } // PgfImage::readPgfHeaderSize
 
-    DataBuf PgfImage::readPgfHeaderStructure(BasicIo& iIo, int& width, int& height)
+    DataBuf PgfImage::readPgfHeaderStructure(BasicIo& iIo, int& width, int& height) const
     {
         DataBuf header(16);
         long bufRead = iIo.read(header.pData_, header.size_);