docs: make MHD picture name unique to allow installation in /usr/share/info

doc/Makefile.am

@@ -33,5 +33,5 @@ microhttpd_TEXINFOS = \
 EXTRA_DIST = \
   $(man_MANS) \
   $(microhttpd_TEXINFOS) \
-  performance_data.png \
-  performance_data.eps
+  libmicrohttpd_performance_data.png \
+  libmicrohttpd_performance_data.eps

doc/libmicrohttpd.texi

@@ -192,7 +192,7 @@ three processes on the same machine).
 
 
 @float Figure,fig:performance
-@image{performance_data,400pt,300pt,Data,.png}
+@image{libmicrohttpd_performance_data,400pt,300pt,Data,.png}
 @caption{Performance measurements for select vs. epoll (with thread-pool).}
 @end float
 

doc/performance_data.eps → doc/libmicrohttpd_performance_data.eps

@@ -1,6 +1,6 @@
 %!PS-Adobe-3.0 EPSF-3.0
 %%Creator: (ImageMagick)
-%%Title: (performance_data.eps)
+%%Title: (libmicrohttpd_performance_data.eps)
 %%CreationDate: (2014-02-19T07:16:22+01:00)
 %%BoundingBox: -0 -0 640 480
 %%HiResBoundingBox: 0 0 640 480