Fix missing include for std::runtime_error

Would fail to compile on Fedora 32 with clang

src/exec.cpp

@@ -2,6 +2,7 @@
 
 #include <memory>
 #include <array>
+#include <stdexcept>
 
 std::string exec(const char* cmd) {
     std::array<char, 128> buffer;
@@ -15,4 +16,4 @@ std::string exec(const char* cmd) {
         result += buffer.data();
 
     return result;
-}
+}