abstract-parser.hh: make the code compile with clang-8.0.0

Without this commit, the build would fail with the following error:

make[2]: Entering directory 'csdiff_build'
[  3%] Building CXX object CMakeFiles/cs.dir/csparser.cc.o
clang++    -O2 -g -DNDEBUG   -Wall -Wextra -fPIC -std=c++11 -o CMakeFiles/cs.dir/csparser.cc.o -c csparser.cc
csparser.cc:393:12: error: call to implicitly-deleted default constructor of 'AbstractParser'
CovParser::CovParser(
           ^
abstract-parser.hh:51:26: note: default constructor of 'AbstractParser' is implicitly deleted because field 'emptyProps_' of const-qualified type 'const TScanProps' (aka 'const map<basic_string<char>, basic_string<char> >') would not be initialized
        const TScanProps emptyProps_;
                         ^
1 error generated.

Closes: https://github.com/kdudka/csdiff/pull/10

Author: kdudka

abstract-parser.hh

@@ -47,6 +47,9 @@ class AbstractParser {
             return emptyProps_;
         }
 
+    protected:
+        AbstractParser() { }
+
     private:
         const TScanProps emptyProps_;
 };