pluma-commands-search: fix warning -Wenum-conversion

[rbuj]

CC=clang CFLAGS="-g -O0 -Wconversion -Wunused-macros -Wunused-parameter" ./autogen.sh --prefix=/usr --enable-debug  --enable-compile-warnings=maximum && make &> make.log && sudo make install

Fix build warning:

pluma-commands-search.c:401:28: warning: implicit conversion from enumeration type 'GtkTextSearchFlags' to different enumeration type 'GRegexCompileFlags' [-Wenum-conversion]
                                                    match_case ? 0 : GTK_TEXT_SEARCH_CASE_INSENSITIVE ,
                                                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Test: find and replace a regex with or without match case.

[raveit65]

Test: find and replace a regex with or without match case.

I see no different in behavior.