oemcommands: fix invalid string find result

Comparison on a std::string::find was against -1, but find returns
an unsigned size_t.  The correct approach is to compare against npos.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I5a5aeff0dad9d2d47eef9bdb258643df8c4464bf
diff --git a/src/oemcommands.cpp b/src/oemcommands.cpp
index b604957..65ef69f 100644
--- a/src/oemcommands.cpp
+++ b/src/oemcommands.cpp
@@ -427,7 +427,7 @@
         }
 
         auto found = path.find(machinePath);
-        if (found == -1)
+        if (found == std::string::npos)
         {
             continue;
         }