Move the getBuildLog implementation to its own implementation file

Keep the header minimal and clean

src/libstore/log-store.cc

@@ -0,0 +1,12 @@
+#include "log-store.hh"
+
+namespace nix {
+
+std::optional<std::string> LogStore::getBuildLog(const StorePath & path) {
+    auto maybePath = getBuildDerivationPath(path);
+    if (!maybePath)
+        return std::nullopt;
+    return getBuildLogExact(maybePath.value());
+}
+
+}

src/libstore/log-store.hh

@@ -11,12 +11,7 @@ struct LogStore : public virtual Store
 
     /* Return the build log of the specified store path, if available,
        or null otherwise. */
-    std::optional<std::string> getBuildLog(const StorePath & path) {
+    std::optional<std::string> getBuildLog(const StorePath & path);
-      auto maybePath = getBuildDerivationPath(path);
-      if (!maybePath)
-          return std::nullopt;
-      return getBuildLogExact(maybePath.value());
-    }
 
     virtual std::optional<std::string> getBuildLogExact(const StorePath & path) = 0;